n57370
This publication presents a historical perspective for informational and educational purposes, is the 

result of independent research, and does not necessarily reflect a position of NSA/CSS or any other U.S. 

government entity.

This publication is distributed free by the National Security Agency. If you would like additional copies, 

please submit your request to:

Center for Cryptologic History

National Security Agency

9800 Savage Road, Suite 6886

Fort George G. Meade, MD 20755

Cover: First Army photo of the bombing of Hawaii, 7 December 1941; the battleship USS Arizona in 

background is on fire and sinking. Signal Corps photo taken from Aeia Heights. 

 Frederick D. Parker retired from NSA in 1984 after thirty-two years of service. Following his 

retirement, he worked as a reemployed annuitant and volunteer in the Center for Cryptologic History. Mr. Parker served in the U.S. Marine Corps from 1943 to 1945 and from 1950 to 1952. He 

holds a B.S. from the Georgetown University School of Foreign Service. 
Pearl Harbor Revisited: 

U.S. Navy Communications Intelligence 1924–1941

Frederick D. Parker 

Series IV: World War II | Volume 6

Third edition 2013

Foreword...................................................................... 5

Introduction .................................................................. 7

Earliest Efforts .............................................................. 8

Role of ONI ................................................................... 9

Early War Plans ............................................................. 11

Japanese Fleet Capabilities and Intentions ............................ 13

Cryptologic Challenge: 

 Navy-Army Cooperation ................................................ 16

Bureaucracy Prevails ...................................................... 18

Status Quo in the Pacific ................................................. 18

A New Attempt at Cooperation .......................................... 19

Japanese Cryptography ................................................... 20

Recovering the “Blue Book” ............................................. 21

Introducing JN-25 .......................................................... 21

Turning Victory into Defeat .............................................. 22

Introducing “Purple” ...................................................... 23

Disarmament: Paying the Price .......................................... 24

Confusing Diplomacy ...................................................... 24

Struggling for Resources .................................................. 25

Planning for War ........................................................... 27

War Games .................................................................. 32

Pacific Buildup ............................................................. 33

National Emergency ....................................................... 33

Circumstances Favor Diplomatic Targets ............................... 34

Information Gaps .......................................................... 35

Cooperation with Allies ................................................... 35

The Move to Hawaii ....................................................... 36

Contents

 
iv

Pearl Harbor Revisited

Support to the Pacific Fleet .............................................. 37

Japanese Intentions Revealed ........................................... 38

Support to Asiatic Fleet ................................................... 38

Japanese Strategy ......................................................... 38

Japanese Navy ............................................................. 39

Diplomatic Messages ...................................................... 42

Warning Messages .......................................................... 43

Reaction ..................................................................... 44

COMINT after the Opening Attack........................................ 46

Conclusion .................................................................. 47

Appendix A: Naval Messages Intercepted between 

 5 September and 4 December 1941 ................................... 49

Appendix B: Summary of

 Diplomatic Messages July–November 1941 ........................... 63

Appendix C: Highlights from COM-14 Daily COMINT Summary ...... 66

Notes ......................................................................... 75

Abbreviations Used......................................................... 83

Notes on Sources ........................................................... 85

Bibliography ................................................................. 88
5 5

activity clearly signaled the creation of a massive carrier strike force with the major naval objective somewhere in the Pacific Ocean far distant from either 

Indochina or the Philippines. 

This comparison of the COMINT record with 

the published material covering the same period will 

benefit not only NSA but also the academic community, which continues to pursue the history of the 

Second World War. Thus whether or not the results 

agree with the literature, particularly if they do not, 

the effort to create an “official” COMINT history is 

more than justified. 

Mr. Parker’s perseverance, diligent research, and 

detailed analysis have made this a significant and 

unique contribution to U.S. COMINT history, U.S. 

military history, and U.S. history. 

Henry F. Schorreck

NSA Historian (Ret.)

[1993]

 

Foreword

Mr. Parker’s monograph is the first in a series 

treating the U.S. Navy’s communications 

intelligence (COMINT) efforts in the Pacific during 

World War II. A second volume, also by Mr. Parker 

(A Priceless Advantage: U.S. Navy Communications 

Intelligence and the Battles of Coral Sea, Midway, and 

the Aleutians), was recently published. 

The series as a whole and this volume in particular are unique in many ways but primarily because 

they represent a closely analyzed, comprehensive 

examination of the COMINT record juxtaposed 

with extensive research into the written history of 

events. Mr. Parker’s work also includes research 

into the Japanese Navy messages which remained 

untranslated until 1945 and undiscovered until now. 

These messages revealed the Japanese Navy plans 

for war with the United States, Great Britain, and 

the Netherlands and the preliminary exercises that 

occurred in the months prior to Pearl Harbor. This 
6

Pearl Harbor Revisited

Radiogram “AIRRAID ON PEARLHARBOR × THIS IS NO DRILL” from 

Commander in Chief of the Pacific Fleet (CINCPAC) to all ships in 

Hawaii area, December 7, 1941. 

17th Naval District, Kodiak, Alaska, General Files, 1940-46, E 91, Dispatches, 

Pearl Harbor; Records of Naval Districts and Shore Establishments, 1784-1981; 

Record Group 181, National Archives
 7

Introduction

systems. Today most intelligence experts would 

call such a practice naive or ill advised. Yet a policy 

requiring OP-20-G cryptanalysts to search primarily for unique cryptographic features of codes and 

ciphers which might later be refined and employed 

by Navy cryptographers was not changed until 1942. 

Coupled with a reluctance to hire civilian trainees, 

this policy seriously delayed the training of enough 

Navy cryptanalysts and linguists to deal with a workload which increased exponentially both in complexity and volume after 1939. Ultimately, the resulting 

shortage of cryptanalysts and Japanese linguists, 

the problem of misplaced priorities, and interservice rivalry issues all contributed to misplacing the 

major focus of the Navy’s cryptanalytic and linguistic 

efforts on Japanese diplomatic messages. The unfortunate result of these circumstances was to postpone 

with fatal consequences an all-out effort on Japanese 

Navy cryptosystems. 

This is not to minimize the value of the pre-Pearl 

Harbor efforts of Navy cryptanalysts and traffic analysts. Even without the messages pertaining to the 

Japanese Pearl Harbor strike force, the magnitude of 

the information they produced pertaining to the Japanese 2nd, 3rd, and 4th Fleets and the Japanese 11th 

Air Fleet was overwhelming. These intimate details 

concerning Japanese intentions, however, were not 

based on messages but on analytic judgments drawn 

from analysis of Japanese Navy communications 

procedures, patterns, and practices. As suspect quantities from a suspect source, they were not accepted by the very commanders in whose service they 

had been developed. The lack of confidence in such 

intelligence made traffic intelligence from the Pacific 

during the last half of 1941 more an elaborate rumor 

than trustworthy source material. Commanders at 

the theater level and in Washington, through lack of 

early training or insight, were not prepared to exploit 

the intelligence provided by this source, particularly 

when the messages themselves could not be read.

In addition to outlining the development of the 

Navy’s cryptanalytic attack against Japanese crypIntroduction

The aspect of the Pearl Harbor disaster which is really surprising is that so 

many people failed to do either the obvious or the sensible things. —Washington Star, 

1 September 1945 

This is the story of the U.S. Navy’s communications intelligence (COMINT) effort between 1924 

and 1941. It traces the building of a program, under the 

Director of Naval Communications (OP-20), which 

extracted both radio and traffic intelligence from foreign military, commercial, and diplomatic communications.1

 It shows the development of a small but 

remarkable organization (OP-20-G, Section/Communications Security) which, by 1937, could clearly 

see the military, political, and even the international 

implications of effective cryptography and successful 

cryptanalysis at a time when radio communications 

were passing from infancy to childhood and Navy war 

planning was restricted to tactical situations.2

 It also 

illustrates an organization plagued from its inception 

by shortages in money, manpower, and equipment; 

total absence of a secure, dedicated communications 

system; little real support or tasking from higher command authorities; and major imbalances between collection and processing capabilities. It explains how, in 

1941, as a result of these problems, compounded by 

the stresses and exigencies of the time, the effort misplaced its focus from Japanese Navy traffic to Japanese diplomatic messages. Had Navy cryptanalysts 

been ordered to concentrate on the Japanese naval 

messages rather than Japanese diplomatic traffic, the 

United States would have had a much clearer picture 

of the Japanese military buildup and, with the warning provided by these messages, might have avoided 

the disaster of Pearl Harbor. 

This story also records what today must be 

ranked as an intensely important interlude when 

the Navy radio/traffic intelligence program deliberately avoided the underlying intelligence of intercepted traffic while exploiting foreign cryptographic 
8

Pearl Harbor Revisited

From 1919 to 1923 the Navy seemed to rely 

almost entirely on the Cipher Bureau. In 1923 the 

Navy apparently felt that the Cipher Bureau had produced neither the desired cryptographic improvements nor the necessary insights into the activities 

of foreign navies, in particular Japan’s growing fleet. 

Accordingly, using shipboard communicators, the 

U.S. Navy in 1923 began an ad hoc effort to listen 

to foreign radio traffic, which its earlier work had 

shown to be potentially vulnerable to penetration 

and exploitation. 

In January 1924, Commander Ridley McClean, 

Director of Naval Communication (DNC) established a research desk within the Code and Signal 

Section with a complement of one officer, Lieutenant Laurance F. Safford, and one civilian, Agnes 

Meyer, both of whom were cryptanalysts/cryptographers. Safford and Meyer conducted research into 

foreign cryptography, organized training in collection and cryptanalysis, developed cryptographic 

systems for naval communications, and arranged 

with the Commander in Chief, Asiatic Fleet (CINCAF), and certain naval district commanders to 

obtain copies of radio intercept of foreign messages. 

The primary goal of the two was to develop cryptographic systems for the U.S. Navy which would 

avoid the weaknesses observed in the cryptographic 

techniques employed by foreign governments. 

Before tackling the problems of penetration 

and exploitation on a regular basis, the Navy had 

to construct an organization which could routinely 

intercept and process foreign cryptographic systems. 

Beginning as a totally decentralized effort loosely 

managed from Washington, DC, collection and 

local exploitation of plain text was controlled by fleet 

and naval district commanders, while Washington 

retained control of the cryptanalytic capability. 

Early in 1924, Commander McClean and 

the Chief of Naval Operations (CNO), Admiral Edward W. Eberle, encouraged the CINCAF, 

Admiral Thomas Washington, to expand radio 

tographic systems, this review also examines other 

interesting episodes overseas and in Washington, 

which included two attempts, one unsuccessful, to 

coordinate Navy and Army COMINT activities, 

efforts to improve fleet communications, and the 

lessons learned and then forgotten about Japanese 

naval communications from Japanese Fleet maneuvers of the l930s. Coordination and cooperation 

between the U.S. Navy COMINT Center on Corregidor and the British Far East Combined Bureau 

in Singapore are briefly described. 

Earliest Efforts 

The origins of the U.S. Navy’s COMINT 

effort prior to 1924 are not entirely clear. 

However, the Navy established a Code and Signal Section possibly with COMINT interests 

as early as 28 July 1916. This small organization 

initially worked against German ciphers during 

World War I. It also tested the security of U.S. 

Navy ship-to-ship and ship-to-shore communications during training maneuvers. During the 

maneuvers of 1917, for example, personnel from 

the section were involved in an overt attempt to 

intercept and exploit U.S. Navy communications 

in order to demonstrate their accessibility to foreign intelligence efforts.3

For some unknown reason, these initiatives 

apparently ended with the World War in 1918. At 

that time the Navy voluntarily consolidated its wartime efforts with those of the War, Justice, State, 

and Postal Censorship Departments, forming a 

single U.S. Cipher Bureau under the War Department. Commanded by Captain Herbert O. Yardley, 

assisted by Captain John W. Manly, the consolidated 

bureau consisted of thirteen cryptographers, twelve 

of whom were women, and an administrator. It was 

supported by eleven student officers, eight stenographers, and fifteen clerks. The Navy was represented 

by Chief Yeoman H. E. Burt.4
 9

Role of ONI

Japanese Imperial Navy Secret Operations Code 

(the Red Book) twice between 1921 and 1927. In 

1931, over the futile objections of Captain Stanley 

C. Hooper, DNC, and Commander J. W. McClaran, 

OP-20-G, the acting Director of Naval Intelligence, 

Captain William Baggaley, returned the entire balance in the account ($65,000) to the U.S. Treasury. 

Why he did this is not clear. It may have been part 

of a Hoover administration economy move. Within 

the Navy the effects were startling. From 1931 to 

1933, Admiral William V. Pratt, CNO, in retaliation 

for this action, ordered that ONI not be shown any 

of the thousands of deciphered messages available 

each year—a policy which must have caused extensive reassignments among ONI’s Japanese linguists. 

In 1929 Lieutenant Commander Laurance Safford 

had advised Hooper that, over a five-year period, 

it had accumulated 10,000 Japanese diplomatic 

messages and, in two years, 2,000 Japanese naval 

messages.6

The early success of OP-20-G in obtaining official and commercial foreign radio traffic and exploiting the knowledge gained from decrypts made posintelligence facilities in his area. As a result of this 

encouragement, the first Navy intercept station 

ashore (Station A) was established the same year 

at Shanghai in the U.S. consulate. It copied both 

naval and commercial traffic ( Japanese and British). 

Admiral Washington was responsible for all aspects 

of the operation including administration, logistics, personnel, and targeting. The Office of Chief 

of Naval Operations, 20th Division of the Office of 

Naval Communications, G Section/Communications Security, or OP-20-G, received all intercept 

logs, including traffic and messages for cryptanalysis. After the codes were broken and the messages 

reduced to plain text, the contents were then sent 

to the Office of Naval Intelligence (ONI) where, 

if necessary, they were translated into English.5

Role of ONI 

This decision to emphasize a communications 

security (COMSEC) objective in exploiting 

foreign communications was to prove costly to OP20-G. It unwittingly conceded to others, notably 

the Office of Naval Intelligence, the responsibility 

for developing and disseminating underlying intelligence, control of language billets and, by thus sowing confusion regarding the nature of communications intelligence, sacrificed much of the long-term 

initiative regarding direction of the overall effort. 

The scope of ONI’s COMINT-related activities 

and the magnitude of the cost of this concession in 

both human and fiscal terms can be illustrated in 

part by the following story. In 1930, the existence 

of a secret fund at Riggs Bank in Washington—at 

one time as large as $100,000—administered by the 

Director of Naval Intelligence (DNI) accidentally 

came to light during the transition of ONI directors. 

This fund was used to buy equipment and material 

in support of DNC’s efforts to intercept and exploit 

foreign radio communications. A strong possibility 

exists that the fund was also used to underwrite the 

costs of stealing, photographing, and translating the 

Captain Laurance F. Safford, cryptanalyst/

cryptographer, Code and Signal Section
10

Pearl Harbor Revisited

and translators which had been inspired by the work 

of Station A (Shanghai), ONI, and the cryptanalysts 

in Washington, Safford and Meyer. 

The years 1926 through 1928, however, saw little 

deliberate progress by the commands in implementing Eberle’s desires, strongly suggesting that the message about COMINT’s value was not widely accepted. 

Rear Admiral George R. Marvell, the Commandant 

14th Naval District (COM-14), did mention radio 

intelligence in his 1928 war plan, but his attempts to 

establish an official intercept site at Wailupe, Hawaii, 

proved abortive. In the Asiatic Fleet area, neither 

Admiral Mark L. Bristol, CINCAF, nor Rear Admiral Sumner E. W. Kittelle, Commandant 16th Naval 

District (COM-16), made any move to enlarge on 

Station A, although Guam and the U.S. legation at 

Peking began to appear in correspondence as possible 

candidates for new sites. 

In the training of intercept operators and cryptanalysts, nevertheless, some real progress did occur 

during this period. In 1926 Ensign Joseph N. Wenger 

was the first officer to undergo training in a cryptanalysis “short course.” Training for officers consisted 

of on-the-job training and semiformal instruction by 

Safford and Meyer. Wenger was followed in the same 

year by Lieutenant Joseph J. Rochefort and Captain 

Leo F. S. Horan, USMC. After completing his class, 

Rochefort was put in charge of the Research Desk 

while Safford performed his required sea duty. Thus 

Rochefort was in charge in 1926–1927 when Meyer 

succeeded in the initial solution of the Red Book 

ciphers and in discovering the “transposition forms.” 

Later the various keys and forms used with a specific 

cipher were also solved by students.7

In 1928 the Navy also established a school for 

enlisted Navy and Marine Corps intercept operators at the Navy Department in Washington, DC. A 

classroom and eight intercept positions were erected 

on the roof of “Main Navy” probably as much for the 

sake of privacy as for the lack of space. Understandably, student graduates became known as the “On 

sible by acquisition of the Japanese Navy’s operation 

code had an immediate as well as far-reaching effect 

on the Navy at large. In March 1926, for example, 

a secret memorandum from Admiral Edward W. 

Eberle, CNO, to all major commands noted that 

current war plans did not “adequately reflect benefits gained from radio intelligence.” Accordingly, 

Admiral Eberle directed that “unit commanders 

both afloat and ashore . . . develop their own plans 

for service of radio intelligence in war.” Eberle’s letter also recommended (1) “intercept and decoding 

units” ashore and afloat; (2) direction finding (DF) 

units in each naval district but primarily in the 12th 

(Headquarters San Francisco) and in the headquarters of the Atlantic and Pacific Fleets; and (3) translators in the 14th Naval District (Hawaiian Islands) 

and in Washington. Clearly this represented a major 

change in perspective regarding radio intelligence 

Admiral Joseph N. Wenger, USN
 11

early war plans

to establish a COMINT collection capability in 

the Pacific were directed at the U.S. Asiatic Fleet. 

It was far from enough, however, and fell short of 

OP-20-G’s goal of an even greater radio intelligence 

capability against the Japanese Navy. U.S. planning 

for this eventuality is treated later. 

In 1930 OP-20-G planners selected the 13th 

Naval District, which included Oregon, Washington, and Alaska, as well as Idaho, Montana, and 

Wyoming, as a prospective location for two new 

intercept sites: one, a large site to cover Japanese 

point-to-point traffic with Europe and China on 

low and high frequencies during wartime; the other, 

a small site in Alaska (“but not in the islands”) to 

cover Japanese ship-to-shore communications in 

both peace and war.12 Because of budgetary restrictions, Admiral Pratt, CNO, was forced to wait until 

May 1932 before directing Rear Admiral E. H. 

Campbell, Commandant 13th Naval District, to 

establish the first of these sites at Astoria, Oregon, 

where the Navy had a DF station providing navigation assistance to commercial vessels.13 Rather than 

build and equip a new site, OP-20-G planners were 

by then reduced to postponing delivery of the new 

equipment and asking Admiral Campbell to accept 

a plan in which a COMINT mission against Japanese targets was to be conducted using idle communications equipment.14 The initial COMINT 

mission was to copy Japanese diplomatic traffic on a 

commercial Radio Corporation of America (RCA) 

circuit between Salinas, California, and Tokyo using 

idle DF receivers which had been tuned to the commercial band.15

In the 14th Naval District, Hawaii, an unofficial 

site established at Wailupe in 1925 was given official 

status in 1931 and authorized one billet by Admiral Pratt, CNO.16 Ineffective because of poor signal 

hearability, the Wailupe site was moved to Heeia in 

1934. After the Japanese attack on Pearl Harbor in 

December 1941, to further improve reception and 

communications, the site was moved to Wahiawa. 

the Roof Gang.” The first class began on 1 October 

1928. Out of twenty students in the first class, seven 

finished. All seven were sent to Guam to open that 

station in 1929.8

 Two classes, 5 and 15, were made up 

entirely of U.S. Marines. The school operated until 

February 1941. Its objective was to train carefully 

selected military radio operators in specialized radio 

communications techniques, particularly Japanese 

intercept, traffic analysis, and simple cryptanalysis.9

Encouraged by these developments, Lieutenant Commander Arthur D. Struble, OP-20-G, 

early in 1929, drafted a letter to Admiral Charles B. 

McVay, CINCAF, which directed the expansion of 

radio intelligence service in his command, including 

activities in China. Significantly, Admiral Charles F. 

Hughes, CNO, who signed the letter, again mentioned that “major decrypting units are planned for 

Washington and Honolulu.”10 Responding to this 

pressure from the Office of CNO, McVay opened 

shore stations at Guam (Station B) and at the U.S. 

legation at Peking, and created a position on his staff 

for a radio intelligence officer. A special cipher was 

supplied to enable this officer to maintain close liaison with OP-20-G in Washington.11 In addition, in 

late 1929 Rear Admiral William D. MacDougall, 

Commandant 16th Naval District, who was subordinate to CINCAF, opened an intercept station at 

a small naval base at Olongapo in the Philippines 

on Subic Bay facing the South China Sea. This site 

(Station C) was destined to move three times in ten 

years in an attempt to find secure operating spaces, 

living quarters, and antenna sites where Japanese 

Navy signals could be heard consistently (Olongapo, 

1930–34; Mariveles, 1934–35; Cavite, 1936–39; and 

Corregidor, 1940–42.) 

Early War Plans 

Because of a continuing perception after the end 

of World War I that war with Japan would 

come sooner or later, the first efforts by the Navy 
12

Pearl Harbor Revisited

Officers, and Station Commanders, to name 

but a few). When you consider that “control” 

was exercised over vast distances using mail 

sent by train and ship by personnel who frequently had no appreciation for the special 

problems faced by those being controlled, the 

wonder is that the system worked at all.17

From the early 1920s, OP-20-G participated in 

U.S. fleet exercises by furnishing cryptographic systems for contending fleet elements, the Battle (or 

Black) Fleet and the Scouting (or Blue) Fleet, and 

by training individual line officers in cryptanalytic 

skills. During the exercises, volunteer cryptanalysts 

often succeeded in penetrating and exploiting the 

opposing fleet’s cryptographic and communications 

systems. The most successful volunteers were officers usually chosen for the assignment by Safford. 

These officers demonstrated a flair for cryptanalysis by solving puzzles Safford placed in the monthly 

Communications Bulletins beginning in July 1924. 

Safford recruited Joseph W. Rochefort, Thomas 

Dyer, Joseph Wenger, E. S. L. Goodwin, Wesley A. 

(Ham) Wright, and Jack S. Holtwick in this way. 

The U.S. fleet problems of 1929 and 1930 dramatically, albeit briefly, called command attention to 

the work of OP-20-G and provided important recognition for the work of cryptanalysts both in developing codes and ciphers for the fleet and in demonstrating decisively the vulnerability of insecure 

communications. U.S. Fleet Exercise #9 in 1929 was 

marked by successful exploitation by Navy cryptanalysts Safford, Rochefort, and Dyer of the Black Fleet 

against both the cryptography and communications 

of the Blue Fleet. Safford, Rochefort, and Dyer read 

all of the traffic of the opposing force (enciphered by 

a cumbersome cylindrical cipher) and made considerable progress in solving the signal cipher as well.18

In his summary critique of the exercise on 15 

May 1929, Admiral Henry A. Wiley, CINC, U.S. 

Fleet, ruefully acknowledged that the successes realized by the decrypting units represented a “serious 

With the exception of closing the sites in 

Peking in 1935 and Shanghai in 1940, the geographic posture of Navy COMINT in the Pacific 

retained this modest form until 1941. Small as the 

collection effort was, however, it is clear that Naval 

Communications had succeeded by 1941 in establishing a radio intelligence organization targeting 

primarily the Japanese Navy in the naval districts in 

the Pacific basin and the Asiatic Fleet. As noted by 

Wenger in 1937: 

It was not an integrated organization, however, but, a number of technically dependent but operationally independent units 

under the technical control of OP-20-G 

but under the military command of the two 

four-star fleet commanders (Pacific and Asiatic). Management control, such as it was, 

was exercised by CNO and administrative 

control by many subdivisions of the Navy 

department and local commander activities (e.g., Bureau of Navigation, Bureau of 

Engineering, District Communication and 

Radio Material Officers, Fleet Intelligence 

Captain Jack S. Holtwick
 13

Japanese Fleet Capabilities

down by expert decryptors [sic].” He further reported that the service cipher used to direct movements 

was unsatisfactory because of the time required to 

encipher and decipher messages. The recognition 

signals were too complicated for quick and effective 

use, though not impossible to memorize or initiate. 

The callsigns were also too long and too easily associated with the user, according to Nulton.22 Despite 

Nulton’s criticisms, the exercises reinforced a growing conviction within the fleet that COMINT was 

a vital tool for commanders, and COMSEC an 

important prerequisite to success in battle.23

Japanese Fleet 

Capabilities and Intentions 

I

n addition to working U.S. fleet exercises to 

make fleet communications more secure, cryptologic personnel overseas copied, analyzed, and, with 

assistance from Washington, exploited radio traffic 

from four Japanese fleet maneuvers between 1930 

and 1935, demonstrating the benefits to strategic 

planning of communications intelligence derived 

from foreign military communications. The stations 

involved comprised Guam, Olongapo, Peking, USS 

Goldstar (AG-12), Los Banitos (Mariveles), and 

USS Augusta (Flagship Asiatic Fleet). Both Augusta 

and Goldstar normally were mobile detachments 

taken from shore stations. At least thirty-three 

operators were assigned operations-related tasks for 

the Japanese maneuvers in 1935, which ran from 

July to September.24

Collectively, these stations intercepted the communications of Japanese ships at sea and from participating Japanese shore stations. The Japanese 

maneuver activity, at its height, typically extended 

from fleet anchorages in Japan to Saipan in the 

Marianas and the Palau Islands east of Mindanao. 

The COMINT reports prepared by personnel at the 

sites, and later consolidated by personnel in Washington who often had message text which supported 

lack of knowledge (of radio security).”19 As a result, 

Safford was directed to “make a cipher which could 

be broken but not allow messages to be read before 

the problem is over.”20

In his memoirs Commander John W. McClaren, who was head of OP-20-G at the time, recalled 

the chaos created the next year when the fleet was 

required to use unfamiliar wartime cryptographic 

procedures, codes, and ciphers during Exercise 

#10.21 Not only was the fleet’s lack of readiness 

amply demonstrated by the resulting confusion, 

but both decrypting units again did quite well 

against the fleet’s codes, ciphers, and communications procedures as well. The Black Fleet decrypting unit consisted of Lieutenant Paul R. Sterling, 

Lieutenant Clarence V. Lee, Lieutenant Frederick 

D. Kime, Lieutenant Frank H. Bond, and Ensign 

William H. Leahy. They were assisted by six yeomen and three messengers. The Blue Fleet unit 

consisted of Lieutenant Llewellyn J. Johns, Lieutenant Wesley A. (Ham) Wright, Ensign R. Bennett, Ensign Lee W. Parke, three yeomen, and one 

marine sergeant. The Black unit recovered the Blue 

Signal Cipher. The Blue unit recovered the Black 

Signal Cipher, the Black Contact Code, and the 

Black Callsign System. 

Fleet Exercise #11 was also held in 1930, 

and again each fleet had a decrypting unit. Johns, 

Brown, Wright, and Parke once again performed 

for the Blue Fleet assisted by three yeomen and one 

radioman, all Navy personnel. In the problem the 

fleets used codes specifically constructed by Safford 

which theoretically could not be broken during the 

exercise. In his critique of 7 April 1930, however, 

commander in chief of the Black Fleet, Admiral 

Louis M. Nulton, reflected on the successes of each 

decrypting unit and leveled considerable criticism 

against designers of the “Recognition Signals,” the 

“Contact and Tactical Report Code,” and, to a lesser 

degree, designers of the callsign system. His critique 

stated that the signal cipher and the control code 

were “simple” and “could have been quickly broken 
14

Pearl Harbor Revisited

coveries” based on traffic analysis, including one 

entitled “Indications of Approaching Hostilities.” 

This prophetic paragraph predicted that “any attack 

by ( Japan) would be made without previous declaration of war or other intentional warning.” In keeping 

with its traffic analysis origins, another finding stated 

that “preparations would be noticeable in increased 

radio activity.” Admiral Upham also recommended a 

plan for observing movements of Japanese merchant 

ships. He believed Japan would try to save as many 

of these vessels as possible by withdrawing them to 

Japan prior to any outbreak of war. Ironically, the 

U.S. Navy did detect such a movement in November 

l941.26 Unfortunately, by the time of Pearl Harbor, 

Admiral Upham was dead, and his report and recommendations lay forgotten in the CNO’s files. 

The 1933 Japanese maneuvers were also noteworthy for two other features which in themselves 

speak volumes on the state of development of 

communications intelligence in the Japanese and 

American navies. Analysts aboard the Goldstar were

aware that the Japanese practiced deliberate electronic countermeasures to prevent the code instruction (CI) unit aboard the Goldstar from successfully 

intercepting their naval maneuver traffic.27 In this 

regard, it is interesting to note that Operation Problem #IV in 1933 at the Naval War College showed 

that the United States could not successfully defend 

U.S. interests in the western Pacific and specifically could not recapture the Philippine Islands or 

hope to maintain a base in these islands. This action 

clearly demonstrated Japanese awareness of the 

value of communications intelligence. On the negative side, however, the final 1933 U.S. naval report 

on these maneuvers was not completed until 1937! 

In May 1937 Wenger, in a personal letter to Holtwick at Station C, commented on his final efforts to 

finish a second report before the end of April. This 

final report was a labor of both love and curiosity. 

Wenger had played a major part in recognizing the 

value of the work done in the field and had collated 

this work into a final report in 1934. In 1937 he 

the field’s conclusions, superbly demonstrated both 

the tactical and strategic value of COMINT and 

established, at least in OP-20-G, the conviction that 

traffic analysis was an equal partner to cryptanalysis. 

Not only did these reports reflect the Japanese fleet’s 

strategic capability to wage a large-scale successful war against the U.S. Asiatic Fleet, but they also 

revealed Japan’s intentions to invade Manchuria, to 

defend the western Pacific in case of a U.S. attempt 

to interfere, and to conduct electronic countermeasures in the event the U.S. attempted to monitor 

fleet communications. The 1930 Japanese maneuver 

was seen by U.S. Navy analysts as a rehearsal for an 

invasion of Manchuria, which actually did occur in 

the following year. The reports also revealed plans 

for the complete mobilization of the Japanese fleet, 

a comprehensive knowledge on the part of the Japanese of the current U.S. war plan against the Japanese fleet, and the unpleasant fact that the Japanese 

Navy was superior in strength to the U.S. Asiatic 

Fleet. Regrettably, the Navy did not see fit to exploit 

this valuable planning asset by regularly tasking and 

funding the resources necessary for its continuation. 

The 1933 report, for example, revealed details of 

Japanese plans to defend the western Pacific from a 

counterattacking U.S. fleet, actual ship movements, 

Japanese war plans against China, and a myriad 

of facts and details about air and sea deployment, 

tactics, communications practices and procedures, 

order of battle, and individual maneuver objectives.25

CINCAF Admiral Frank B. Upham was particularly 

impressed by the efforts of the COMINT analysts, 

whose work was based entirely on traffic analysis, 

since the Japanese Navy’s operational code (the Blue 

Book) had not been recovered by the time of the 

exercises. Not only did he visit Station C to personally compliment the men, telling them that one 

day their work would be of tremendous importance 

to the nation, but he prepared a unique endorsement for the report. His endorsement, forwarded 

to Admiral William H. Standley, CNO, on 20 June 

1934, contained several significant “COMINT dis-
 15

Japanese Fleet Capabilities

greatly alarmed U.S. officials because it contained 

the Nagato’s new top speed, which was in excess 

of twenty-six knots, the same as four new Kongoclass battle cruisers and considerably in excess of 

the twenty-four-knot top speed currently planned 

for the redesigned U.S. battleships North Carolina 

and Washington. By inference the Nagato’s speed 

would be the prospective speed for other battleships 

being modernized and the minimum speed for new 

battleships of the Yamato class. As a direct result of 

this message, U.S. naval officials raised the required 

speed of modernized U.S. battleships to twenty-seven knots and of new vessels to twenty-eight knots.30

wanted to evaluate the traffic analysis results against 

Blue Book recoveries not available earlier to see if 

the messages contained any unique information.28

Thus, largely because of a lack of manpower, five 

years elapsed between the event and the Navy’s final 

COMINT report on a significant Japanese fleet 

exercise which revealed many unique Japanese Navy 

capabilities.29

Another important contribution to the U.S. 

Navy’s efforts to determine the capabilities of the 

Japanese Navy occurred in 1936 when the cryptanalysts and linguists in Washington translated a 

message giving the results of the Japanese battleship 

Nagato’s postmodernization trials. This message 

Japanese battleship Yamato
16

Pearl Harbor Revisited

fic on diplomatic circuits had created an immense 

workload for Navy cryptanalysts. This forced the 

Navy to realize that it could no longer handle 

both targets and to seek a division of effort with 

the Army, with which it would furnish intercepted 

traffic until the Army could develop its own collection capabilities.33

Not willing to give up all diplomatic communications, however, the Navy proposed that the Army 

analyze all counterpart Army radio communications 

and all diplomatic radio communications except 

for those of the four major naval powers, England, 

France, Italy, and Japan. This arrangement, the Navy 

claimed, would help it reduce an estimated two-year 

time lag in breaking the Japanese Blue Book.34

For a number of reasons, negotiations were not 

immediately fruitful. A primary cause for the lack 

of progress in the negotiations was that Army intercept sites, when established in the United States or 

even those existing in the Philippines, would not be 

able to hear low-power military radio transmissions. 

This unpleasant fact made the Navy’s proposition 

partially irrelevant except in China. There Station 

A could and did irregularly intercept both Chinese 

and Japanese ground forces communications, which 

were provided to Army analysts. Talks continued 

without resolution until 1933, when a tentative 

position was developed for presentation to the Joint 

Army-Navy Board under a much broader heading, 

“Joint Effectiveness of Army and Navy Communications Systems.” The joint proposal encompassed 

not only COMINT but communications and communications security matters as well. Possibly in 

return for its promise of cooperation on COMINT 

target distribution as outlined in 1931, the Army 

obtained concessions from the Navy in several vital 

areas including training intercept operators and in 

preparing a COMSEC Annex to Army War Plans. 

In addition, the Navy agreed to provide training 

for enlisted communicators and communications 

officers.35

Cryptologic Challenge: 

Navy-Army Cooperation 

For several years in the early 1930s, the U.S. 

Navy was the only source of intercepted traffic from Japanese communications. After the Army 

obtained an intercept capability against diplomatic 

targets sometime in 1935, a “friendly rivalry” developed as both services attempted to intercept and 

read as much as possible “to gain credit” for the 

intelligence.31 In the late 1930s the Navy discovered 

that the same Japanese consulate which had twice 

yielded the Red Book in the early 1920s was also 

a likely source for “effective and reserve” ciphers 

and keys for all current diplomatic systems except 

the two machine systems A and B. Once again, by 

“borrowing” them the Navy was able to provide the 

Army and the Asiatic Fleet with Japanese diplomatic ciphers and keys for manual systems before they 

came into use. 

The decade of the 1930s also witnessed a 

resurgence of U.S. Army interest in cryptanalysis. In 1930, after the collapse of Yardley’s New 

York “Black Chamber,”32 William F. Friedman was 

tasked to create an Army cryptologic capability in 

the office of the Chief Signal Officer. Starting with 

four civilian students whose names have become 

bywords in the U.S. cryptologic community—Frank 

Rowlett, Solomon Kullback, Abraham Sinkov, and 

John Hurt—Friedman began the slow and difficult 

training process which would ultimately lead to the 

compilation of War Department codes and ciphers 

and the solution of foreign military and diplomatic 

codes and ciphers. 

It was to this embryonic work force that the 

Navy turned in 1931 for help against two cryptographic targets which at the time almost completely occupied OP-20-G’s efforts: Japanese diplomatic and naval communications. The introduction of 

the Blue Book, as the Japanese Navy Operations 

code was known, in February 1931 (replacing the 

Red Book) and an unexpected surge in cipher traf-
 17

Cryptologic Challenge: Navy-Army Cooperation

tem of cooperation on COMINT targets almost 

collapsed within days of its tentative approval when 

it was disclosed by the Army that, inexplicably, 

the State Department had completely rejected the 

proposal insofar as it pertained to the Army’s collecting diplomatic communications. According to 

internal Navy correspondence, Army negotiators 

from the office of the Chief Signal Officer discussed 

the proposed division of effort with the chief of the 

Army’s War Plans Division, who informed the State 

Department. State’s rejection of the plan was reported in a memorandum to DNC by OP-20-G on 10 

April 1933. Despite State objections, however, some 

degree of cooperation between the Navy and Army 

seemed assured.36

In 1933 the official aim of both the Navy and 

the Army in the negotiations could be summed up 

in two words: “cooperation” and “uniformity.” Uniform communications, uniform censorship rules in 

wartime, uniform authentication systems, and common recognition signals for aircraft, local defense 

forces, and defense districts were goals which motivated both sides. Since the Navy already had such 

tools in place within its framework of naval districts 

and the Army lacked such a structure, it clearly 

made sense for the Army to consider building on 

the Navy’s experience. 

Regarding COMINT matters, however, joint 

agreements were harder to resolve. The fragile sysWilliam F. Friedman was tasked with creating an Army cryptologic 

capability in the office of the Chief Signal Officer in 1930. 

Seated: A. J. McGrail, W. Preston Corderman, Friedman 

Standing: Mark W. Rhoads, Solomon Kullback, John B. Hurt, 

Edward J. Vogel, Frank B. Rowlett, Abraham Sinkov 
18

Pearl Harbor Revisited

present, responded by recommending to the secretary of war that nineteen separate joint committees 

be established to increase “the joint effectiveness 

of Army and Navy Communications Systems ....” 

Among the committees which were to report back 

to the Joint Board for final action were two which 

pertained directly to radio intelligence: (1) Radio 

Intercept and Radio Intelligence Problems, chaired 

jointly by Major Spencer B. Akin of the Signal 

Corps, and Commander Howard F. Kingman and 

(2) Communication and Radio Intelligence Development Board, chaired jointly by Major Akin and 

Commander F. D. Pryor (Commander Kingman 

was a member). The following day, 14 July 1933, 

the secretary of war, George H. Dern, and secretary 

of the navy, Claude A. Swanson, approved the Joint 

Board’s recommendation.38 Unfortunately, with 

their cryptanalytic workload once again in hand and 

despite the existence of the committees, Navy officials were unable to arrive at a satisfactory consensus 

with the Army on COMINT cooperation, and the 

subject appears to have languished until circumstances in 1940 once again demanded attention. 

Status Quo in the Pacific 

Until the end of 1938 the Japanese maintained 

a cryptographic status quo which enabled U.S. 

Navy cryptanalysts to live in the best of worlds. 

After the Blue Book System was reconstructed 

by OP-20-G, it was a relatively simple matter for 

the better part of five years to follow the activities of the Japanese Navy. As one observer noted 

during the 1930s in the Philippines, “Japanese 

traffic was everywhere you looked.” There were so 

many options available that a selection process was 

required to control collection.39 In Washington, 

Op-20-G’s interest in Japanese diplomatic traffic 

until 1938–39 remained almost a purely technical one—that is, solving ciphers and recovering 

keys simply for their cryptographic value. The one 

exception was the support provided overseas cryptBureaucracy Prevails 

With the Blue Book about to be solved, but 

undoubtedly aware that much work in defining mutual areas of interest remained to be done, 

the parties were not about to be deflected from their 

main military goal by a civilian State Department. 

A memorandum for the Joint Army-Navy Board 

was approved by Admiral Pratt on 24 April 1933. 

Attached to the memorandum was a list of twelve 

joint studies including the recommendation that “a 

joint study should be made in regard to radio intercept and radio intelligence problems ... (because of 

manpower shortages) division of the work should 

be agreed upon and exchange of information, in 

outlying districts particularly, should begin without delay.”37 On 13 July 1933, the Joint Board, with 

General Douglas MacArthur as senior member 

Major General Spencer B. Akin, U.S. Army 

Signal Corps, joint chair of Radio Intercept 

and Radio Intelligence Problems Committee
 19

New Attempt at Cooperation

Before the study could be undertaken, the Army 

General Staff ordered the Signal Corps to copy the 

diplomatic circuits of Japan, Germany, Italy, and 

Mexico.41 Although it meant wholesale duplication 

of collection, this directive left little room for the 

two departments to negotiate (no doubt to Safford’s 

immense relief ) and led eventually to the recommendation of August 1940 in which the U.S. Navy 

became responsible for deciphering and translating 

Japanese diplomatic and consular service messages 

on odd days of the month and the Army on even 

days (see Chart A). This narrow and highly simplified arrangement at least relieved Safford of the 

specter of two conflicting translations of the same 

message being delivered to the president. It did not, 

however, as will be seen, relieve the Navy’s cryptanalytic and linguistic workload, particularly in 1941 

analysts, particularly at Station C, who were supplied Japanese cipher and key recoveries for their 

value in developing COMINT in support of the 

Asiatic Fleet. Station C’s responsibilities included 

keeping CINCAF informed of developments in 

diplomatic as well as naval messages copied by 

Peking, Guam, and the Philippines.40

A New Attempt at Cooperation 

During 1938–39, U.S. successes against both the 

naval and diplomatic targets began to unravel 

as the Japanese changed their long-standing cryptographic systems. These developments brought the 

two U.S. military departments back to the bargaining table in mid-1940. As usual, both sides agreed to 

go their own way on international commercial and 

counterpart communications. Regarding diplomatic 

communications, General Joseph O. Mauborgne, 

Chief Signal Officer, U.S. Army, proposed an elaborate study to determine which targets could be heard 

by the individual stations of each service. According 

to Mauborgne’s proposal, responsibility would be 

assigned according to hearability, frequency, time of 

day, type of transmission, and, in the case of duplication, preponderance of copy without regard for the 

underlying value of any intelligence to the intercepting agency. 

Convinced that the OP-20-G work load was 

already excessive, Safford originated several appeals 

to Rear Admiral Leigh Noyes, DNC, between 

July and September concerning the pitfalls of this 

approach. In October 1940, for example, he advised 

Noyes that the Navy did not want to do German, 

Mexican, and Italian traffic. He also said that the 

Signal Corps had little to do if it did not copy highpowered diplomatic transmitters since its stations 

could not hear the relatively low-powered military 

radios. He advised Noyes that the Navy should 

relinquish the entire diplomatic target rather than 

agree to the proposed Mauborgne scheme. 

Major General Joseph Mauborgne, Chief 

Signal Officer, U.S. Army
20

Pearl Harbor Revisited

York. The Army too deserves credit and praise for 

its work against high-level machine systems used in 

enciphering Japanese diplomatic messages. 

From 1924 to 1940, U.S. cryptanalysts adopted 

a system of color designations for certain high-level 

Japanese cryptographic systems. The Japanese diplomatic machine ciphers were designated Red for the 

A machine and, in 1939, Purple for the B machine 

which replaced it at many embassies. In 1939 a 

naval attaché machine cipher was introduced. It was 

designated Coral by the U.S. and was in use until 

1945.43 The Japanese Navy’s main operational code 

was designated Red until 1930, Blue until 1938, and 

Black until 1940, when its designation was changed 

to JN-25, the Fleet General Purpose System.44

The Japanese Navy also employed several other 

cryptsystems to conduct its business which were not 

swept into the U.S. system of color designations. 

as the crisis between Japan and the United States 

deepened and the number of diplomatic messages to 

and from Japan increased. The recommendation was 

nevertheless approved on 3 October 1940.42

Japanese Cryptography 

Despite a prevailing shortage of cryptanalytic manpower between 1924 and 1941, the 

U.S. Navy’s efforts against Japanese naval codes 

and ciphers were marked by some brilliant successes. Much was due to the inspired work of people assigned to OP-20-G, such as Safford, Agnes 

Driscoll (née Meyer), Dyer, Wright, and Holtwick. 

Some of the success, however, must be attributed 

to the ONI, which three times in this period “borrowed” Japanese naval and diplomatic manual codes 

and ciphers from the Japanese consulate in New 

Chart A 

Army and Navy Sites Authorized to Intercept Diplomatic Traffic, August 1940

Army Navy

Site location

Site 

designator

Number of 

collectors Site location

Site 

designator

Number of 

collectors

Fort Monmouth, 

NJ 1 19

Winter Harbor, 

ME W 8

Presidio, CA 2 9

Amagansett, 

NY G 4

Fort Sam 

Houston, TX 3 14

Cheltenham, 

MD M 20

Corozal, CZ 

[Panama Canal 

Zone] 4 20 Jupiter, FL J 4

Fort Shafter, HI 5 19

Bainbridge 

Island, WA S 12

Fort Hunt, VA 7 24 Heeia, HI H 8

Totals 6 105 6 56
 21

Introducing JN-25

Recovering the “Blue Book” 

At the end of October 1938, however, without 

warning the Japanese Navy changed its operational code. Why the Japanese chose this moment to 

make the change is unknown. Perhaps they feared 

their old system had been penetrated, or perhaps 

this was the beginning of a cycle of changes. The 

change replaced the Blue Book, which had been 

used since 1931, with the Black Code. 

The outgoing Blue Code was never used without a cipher to be stripped off before the code could 

be reconstructed. Navy cryptanalysts Safford, Dyer, 

and Driscoll solved the Blue Code in 1933, making possible the important successes against the 

Imperial Fleet exercises in 1934 and 1935. Their 

success had followed what was possibly the most 

difficult cryptanalytic task ever undertaken by the 

United States up to that time. In Safford’s opinion, 

Driscoll’s work in solving the system may have been 

even more brilliant than the Army’s subsequent 

solution of the Purple machine because “there were 

no cribs or translations to help out.”51 The introduction of IBM “tabulating machines” against the Blue 

Book was also a major advancement at the time.

Introducing JN-25 

Two additional codes augmented the Japanese 

Black Code beginning on 1 June 1939: the 

“Flag Officers Code,” which saw very limited use 

and was never broken, and a five-digit enciphered 

general-purpose code given the designator “JN-25.” 

The Flag Officer’s Code was one of Hawaii’s principal tasks until mid-December 1941. 

The JN-25 system required three books to operate: a code book, a book of random numbers called 

an additive book, and an instruction book. The 

original code book contained some 30,000 five-digit 

numbers which represented kana ( Japanese script) 

particles, numbers, place-names, and myriad other 

At OP-20-G, for example, one worker decrypted 

all messages in the Japanese navy-merchant vessel 

liaison code.45 U.S. cryptanalysts read the code in its 

entirety from the fall of 1939 to the tenth of August 

1941.46 Six other Japanese naval systems were intercepted regularly. Two of these—an auxiliary ship 

cipher and a minor general-purpose system—were 

not worked. A third, an intelligence code, was considered of little importance after its contents were 

discovered, and it was ignored. The three remaining 

systems were worked intensively. They were the Japanese naval administrative system, a materiel system, 

and the fleet general-purpose system. The administrative and materiel systems had similar encipherment forms, and both encipherments were broken 

from time to time. When this occurred, two workers 

were assigned to recovery of the underlying codes. 

Success in the administrative system led to a limited 

capability to solve the general-purpose code. The 

materiel code was worked during the spring of 1940 

in an unsuccessful attempt to learn details about the 

performance characteristics of the battleships Yamato and the Musashi, superbattleships built in violation of existing treaties, which were launched in 

1941 and 1942, respectively. Regrettably, all recoveries on Japanese naval systems before Pearl Harbor 

yielded cryptanalytic technical information rather 

than current intelligence.47

In the grips of a rapidly expanding workload, 

the limited number of skilled U.S. cryptanalysts 

and linguists made it impossible to produce current 

intelligence except in the diplomatic field.48 The 

explosive growth of Japanese diplomatic and naval 

cipher traffic (1,200 percent growth between 1930 

and 1935)49 continued in both volume and numbers of systems throughout the 1930s. By the end 

of 1942, the Japanese Navy employed fourteen different minor systems which generated over 40,000 

messages per year in addition to messages obtained 

from the general-purpose system which, by November 1941, had reached 7,000 messages per month.50
22

Pearl Harbor Revisited

1940, U.S. cryptanalysts had recovered the system 

of text additives, two systems of keys, and the actual 

code groups for the numbers 000 through 999. At 

this point the only factor which seemed to prevent 

complete exploitation of JN-25 was lack of manpower. Out of the total cryptanalyst population in 

Washington at this time (thirty-six in December 

1940), only from two to five people could be spared 

to work on this still unreadable system.54

Turning Victory into Defeat 

On 1 December 1940, probably before Washington’s work could be distributed to the field stations, Japan canceled the Able code which had been 

used since 1 June 1939. This action completely frustrated any hope of code exploitation by 1 January 1941. 

The new code was named JN-25 “Baker.” It proved to 

have several unfamiliar features in key generation as 

well as new and larger code and additive books. For 

the next two months, however, until 31 January 1941, 

many messages were intercepted in which the Japanese employed Able code additives already recovered 

in Able Five. OP-20-G lost no time in exploiting this 

cryptographic blunder by placing the entire Corregidor 

effort and most of the Washington effort on the current cipher and code recoveries.55

With the progress made on recovery of the new 

code values, U.S. officials believed that the combined 

efforts of all units would again bring the system close 

to the point of reading current traffic by early summer 1941. Code recovery continued to progress well. 

Throughout the summer and fall of 1941, new discoveries about the nature of the code were routinely 

committed to a Registered Intelligence Publication 

(RIP) and given wide if slow-moving distribution to 

the field units. 

The actual reading of current Japanese messages before Pearl Harbor, however, was not to be. 

U.S. cryptanalysis of the ciphers had outstripped 

the U.S. capability for code recoveries. That is, OPmeanings. A key characteristic of this system was 

that, when the digits in a group were added together, 

the total was always divisible by three. The book of 

random numbers consisted of 300 pages, each of 

which contained 100 numbers on a 10 × 10 matrix. 

These numbers were used as additives—they were 

added to the code groups digit by digit without the 

carryover used in customary addition—thus enciphering the code. The instruction book contained 

the rules for using the aperiodic cipher. The number of each page and the number of the line on the 

page where the selection of additives began served as 

“keys” which were included in each message at the 

beginning and end. This code subsequently became 

the most widely distributed and extensively used of 

all of Japan’s naval cryptosystems.52

Using improved IBM card-sorting equipment 

and newly developed analytic techniques and noting 

similarities to an earlier four-digit “S” system stolen 

from a consulate, Agnes Driscoll and her colleagues 

were soon stripping off daily keys and additives in 

the Able, or first cipher, and slowly reconstructing 

the code.53 After investing a year in attempting to 

understand its components, OP-20-G put aside all 

work against the current JN-25 cipher during the 

summer and fall of 1940 in favor of slow but steady 

progress toward actual reading of the underlying 

code. After keys were recovered on each new cipher, 

the traffic itself was filed for later study. 

Though they were working on year-old traffic, 

the cryptanalysts recovered a segment of the Able 

code which led to discovery of pattern messages, 

such as medical reports, and stereotyped messages 

containing noon positions for convoys. On 1 October 1940, the Japanese introduced the fifth Able 

cipher (Able Five). It was quickly diagnosed by 

OP-20- G analysts. Once the new keying system 

was understood, Washington policymakers decided 

that all units, including Hawaii, should begin working on the current cipher in the hope that by January 

1941 the first JN-25 message of the new year would 

be read on the same day it was sent. By December 
 23

Introducing “Purple”

intercepted diplomatic messages and the first prototype deciphering machine. 

The prolonged delay resulted primarily from 

the complexity of the new Japanese machine. Its 

impending introduction had been anticipated for 

several months in 1938, when distribution of the 

equipment was noted in intercepted messages. Only 

the more important Japanese embassies received the 

new machine, including Rome, London, Washington, and Berlin. Those which did not receive the 

Purple equipment continued to use the Type A 

machine. In fact, when maintenance was required on 

the new equipment, a Japanese embassy frequently 

reverted to the Type A machine.57

Recovering from their initial confusion, U.S. 

cryptanalysts quickly began to exploit the new 

machine despite its complexities. By 10 April 1939, 

Frank B. Rowlett and Robert O. Ferner had produced partial texts based on similarities between the 

A and B systems.58 However, on 1 May 1939, apparently recognizing the vulnerability of their new 

system, the Japanese introduced significant complications to the recovery process. By 27 November 

20-G and Corregidor (as well as London and Singapore) had not recovered enough of the basic code, 

and JN-25 decrypts could not be produced in time 

to play a part in U.S. and policy or military decisions during this crucial period. Thousands of intercepted Japanese Navy messages in JN-25 were not 

exploited because, as a result of manpower shortages 

and higher priorities, the underlying code values 

remained unrecovered.56

These proved to be costly factors indeed, 

because analysts at Hawaii, Corregidor, and Washington never discovered the vital information contained in the untranslated messages. We now know 

that they contained important details concerning 

the existence, organization, objective, and even the 

whereabouts of the Pearl Harbor Strike Force, the 

Japanese Navy’s First Air Fleet. Hidden in these 

messages was the full magnitude of the enterprise 

planned by the Japanese to begin on 7–8 December 1941. Had these messages been read on a current basis, it is possible—even probable, given the 

analytic skills so evident in these centers—that the 

early course of the war would have been significantly altered. Unfortunately, most of the U.S. Navy’s 

cryptanalytic effort was devoted to another Japanese 

cryptographic problem: recovering the daily cipher, 

translating the texts, and reading the Japanese diplomatic messages. 

Introducing “Purple” 

I

n February 1939, only a few months after discovering the JN-25 and Black Code on naval 

communications, another shock struck the US. 

cryptanalytic community when the Japanese introduced the Type B machine on their high-level diplomatic circuits. Known as the “Purple” machine, it 

was eighteen months before the efforts of William 

Friedman’s staff at the U.S. Army’s Signal Intelligence Service (SIS) and the Navy Yard Machine 

Shop succeeded in producing full translations of 

Thousands of intercepted 

Japanese Navy messages in JN-25 

were not exploited because, as a 

result of manpower shortages 

and higher priorities, the 

underlying code values 

remained unrecovered.
24

Pearl Harbor Revisited

States, England, and France grew more intransigent, 

a realization developed in Navy circles that budgetary 

decisions since the end of World War I, and particularly since 1929, had almost crippled the U.S. fleet. The 

most severe suffering was felt in manpower-intensive 

activities such as the COMINT effort. While Germany and Japan openly rebuilt their military establishments during the depression years, the U.S. Congress, 

preoccupied with disarmament and rebuilding the 

nation’s economy, consistently imposed harsh fiscal 

constraints on the Navy. In the name of disarmament, 

Congress called for reductions in both capital expenditures and manpower. For OP-20-G the manpower 

restrictions had such a severe impact that Safford was 

to feel their effects up until the eve of the Japanese 

attack.61 Moreover, the lack of money for investment 

meant that not only the operational structure but also 

the support framework would suffer. For example, a 

secure electrical communications network could not 

be built. This meant a continuation of those interminable delays in the exchange of cryptanalytic data and 

intercepted traffic because surface shipments from and 

to China and the Philippines (and Hawaii) customarily took weeks and even months. Finally, as if these 

problems were not enough, Congress, in 1934, passed 

the Communications Act, which declared communications intelligence an illegal activity.62

Confusing Diplomacy 

The same sort of frustrating inconsistencies appeared in U.S. foreign policy toward 

China and Japan with far more serious consequences, particularly in their impact on Navy 

planning. Until 1939, the U.S. government followed a pattern of conflicting policies regarding 

the two nations. Committed on the one hand to 

an Open Door Policy toward China, the U.S. conversely recognized in 1908 and again in 1917 that 

Japan had special rights and interests in eastern 

Asia because of its “territorial propinquity.” The 

Lansing-Ishii Agreement of 1917, in fact, specifi1940, however, U.S. Army analysts produced two 

translations which represented the first solutions 

to the B Machine.59 OP-20-G played a minor but 

important role during this period. Purple analog 

machines, based on wiring designs developed by the 

Army, were made at the Navy Yard in Washington, 

DC, and distributed to the War and Navy Departments, that is, to SIS and to OP-20-G, and by mid1941, to Admiral Thomas C. Hart, CINCAF. The 

British in London were given equipment originally 

intended for Hawaii. After the Army solved the system, Navy and Army cooperated in recovering the 

daily changing keys. Messages from both the Purple 

and Red machines were known as MAGIC.60 Once 

the Purple machine became readable and the need 

for translated current Japanese diplomatic messages became urgent, the War Department requested 

additional Navy assistance in the form of cryptanalysts and linguists. 

Disarmament: Paying the Price 

The events of 1938 and 1939, which virtually devastated—if only temporarily—the U.S. cryptanalytic efforts against Japan, were only the latest in 

a series of setbacks and disappointments which had 

begun with the decade. As the war in Europe expanded and Japanese behavior toward China, the United 

Congress, in 1934, passed the 

Communications Act, which 

declared communications 

intelligence an illegal activity.
 25

Struggling for Resources

Forced by domestic economic considerations 

to cut back on military spending, the United States 

continued to adhere to arms limitations agreements and self-imposed building moratoriums well 

into the 1930s while the Axis powers skillfully circumvented them by modernization programs and 

new construction. By 1939 both the U.S. and British navies had fallen behind the Japanese Navy, not 

just in numbers of modern vessels but particularly 

in the technology of naval architecture and naval 

armaments, ship design, hull speeds, torpedoes, and 

the caliber of ships’ guns. One bright spot during 

this period, as noted earlier, was that the United 

States was quick to react to the COMINT-derived 

information concerning battleship speed revealed 

in late 1936. Regrettably, there was no corresponding move to upgrade the Navy’s COMINT 

program. 

Struggling for Resources 

Throughout this period, while struggling to 

establish a presence in the Navy, OP-20-G had 

failed to find a method of assuring a steady supply 

of manpower. Until the 1930s, when students began 

to graduate from the intercept operators’ school in 

Washington, OP-20-G drew its enlisted manpower 

from two career fields—communications specialists and yeomen. Accordingly, since these students 

were not given career field designators appropriate 

to their unique role in naval communications (i.e., 

intercept operators), it was not unusual for almost 

twenty years to see correspondence from OP-20-G 

to naval district commanders rescuing these people 

from being assigned duties in their ostensible fields. 

Until late 1941, the number of intercept operators in the Pacific was never very high, thus making 

their daily availability a matter of some concern to 

the resident officer-in-charge (OIC). In the Philippines in 1933, for example, it reached an unusually large total of eighteen men whose orders of 

cally recognized Japan’s special position in Manchuria and on the Shantung Peninsula. Moreover, 

until 1941 the U.S. consistently supplied Japan 

with the war materials necessary to undertake 

and sustain operations not only against China 

but against the Netherlands and France as well. 

At the same time, the United States maintained a 

naval rivalry with Japan which, because of various 

factors, had already begun to tilt in Japan’s favor 

following the end of World War I. 

To the U.S. Navy these policies contained serious 

strategic implications. In the early 1920s the United 

States was faced with the unpleasant prospect not 

only of the continuation of a prewar Anglo-Japanese 

alliance with unfavorable balance of power implications, but also with the equally distressing prospect 

of a superior Japanese fleet in the Pacific, occupying the German islands which lay astride U.S. lines 

of communication to Australia and making defense 

of the Philippines virtually impossible. Aided by 

Canada and Australia at the Washington Conference in late 1921, the U.S. succeeded in replacing 

the Anglo-Japanese alliance with a four-power 

treaty with Britain, France, and Japan. This treaty 

unfortunately limited U.S. and U.K. base building in the Pacific in return for reluctant Japanese 

acceptance of apparently unfavorable ratios in naval 

strength. Although not at first seen as an advantageous treaty for Japan, several factors conspired to 

make it so. Among these were an obsolescent British dreadnaught fleet which effectively eliminated 

the British Asiatic Fleet as a force; a moratorium on 

battleship construction which saw the United States 

scrap twenty-eight vessels including eleven capital 

ships in various stages of completion;63 a U.S. commitment to a two-ocean navy which meant that not 

all new ships joined the Pacific Fleet; and the basebuilding restrictions of the four-power treaty. Collectively these measures left Japan in a position of 

local superiority and in a dominant position regarding the coast and approaches to China, the treaty 

notwithstanding.64
26

Pearl Harbor Revisited

staff assignment the RI officer worked with an ONI 

officer trained in the Japanese language who was usually senior in grade, a situation which, under certain 

operational circumstances, could prove awkward. To 

forestall any rank-generated problems, an agreement 

was struck in the Asiatic Fleet which placed final 

COMINT responsibility on the OIC of Station C.66

The method of selecting an officer as a trainee 

in cryptanalysis was slightly different though colored with the same sort of influences and priorities 

found among enlisted men. Between 1920 and 1940, 

a career as a naval line officer (e.g., gunnery officer) in 

the fleet was the prime pathway to success for Academy graduates. Naturally, the fleet had first priority 

on any and every line officer. Normally, an Academy 

graduate spent his first seven years at sea, two years 

ashore, then three years at sea.67 Tours in gun turrets on battleships and cruisers usually led to more 

responsible positions on board ship (such as navigaassignment to the 16th Naval District carried the 

caveat “only for intercept or RI [radio intelligence] 

research work.” Ordinarily, from 1930 to 1936, when 

the first DF site opened at Sangley Point on Luzon, 

the average assigned strength was only nine. Lack of 

numbers, however, did not reduce either the amount 

or nature of the work required. In 1937 a fifteenman work force at Station C was divided into four 

three-man watches. “Other duties” for this group 

included electronic and typewriter maintenance, 

translation, and traffic indexing.65

The number of officers involved in radio intelligence in the Philippines was even smaller. For the 

eight years between 1934 and 1941, it was typically 

limited to two Washington-trained cryptanalysts. To 

extend their presence in the fleet for as long as possible, they would usually serve two tours: the first as 

OIC of Station C, followed by the job of radio intelligence officer on the staff of the Asiatic Fleet. In the 

Chart B

Radio Intelligence Officers, Philippines Intercept 

Station (Station C), 1934–1941

Name

Officer in Charge, 

Station C

CINCAFa

Radio 

Intelligence Officer

Lieutenant Thomas B. Birtly July 31–Oct 31

Lieutenant Bern Anderson Apr 32–Oct 32

Lieutenant Joseph N. Wenger Oct 32–Jun 34

Lieutenant JG Thomas A. Huckins Aug 34–Nov 35

Lieutenant JG E. S. L. Goodwin Jun 34–Nov 35 Nov 35–Mar 37

Lieutenant JG Roy S. Lamb Nov 35–unknown unknown–Dec 37

Lieutenant Jack S. Holtwick unknown–Nov 37 Dec 37–Mar 39

Lieutenant J. A. Williams Nov 37–Feb 39 Mar 39–Jan 40

Lieutenant Jefferson R. Dennis Feb 39–Jan 40 Jan 40–unknown

Lieutenant Bernard F. Roeder Jan 40–unknown unknown–Oct 41

Lieutenant Rudolph J. Fabian unknown–Feb 42

Lieutenant J. M. Leitwiler Feb 42–Apr 42

 aCommander in chief, Asiatic fleet
 27

Planning for War

perform in wartime, particularly during a war with 

Japan. While it has proven virtually impossible to 

trace, the following discussion of strategic planning 

strongly suggests that the impact of a new naval 

strategy in 1940–41 on COMINT resources in the 

Pacific was pervasive, overpowering, and largely 

negative. The Japanese scenario had existed since 

the turn of the century when the United States, 

after its war with Spain, found itself in possession 

of many islands in the Pacific Ocean, most notably 

the Philippines and Guam, which it could neither 

administer nor adequately defend. The military 

aspects of the situation called for close cooperation 

between the Army and Navy and in 1903 led to creation of the Joint Army-Navy Board, usually known 

as the Joint Board. From its inception the board 

concerned itself with prospects of war with Japan, 

particularly after Japan emerged victorious from the 

Russo-Japanese war in 1904–1905. A fundamental 

assumption by the board was that the Philippines 

would always be Japan’s first wartime objective. 

Adopting a series of colors to identify its plans, 

the board developed the first Japanese war plans 

(Orange) in 1904–1905. The usual pattern was for 

the joint plan to be augmented by individual service 

plans which were constantly reviewed and refined 

each year depending on military necessity, the 

moods of Congress, and the international situation. 

As the plans grew in complexity, the service plans 

themselves were augmented by individual service 

plans such as Naval Communications and Naval 

Intelligence. 

The version of the Navy’s Orange War Plan 

which was current in 1941 actually had its inception 

in May 1929 as WPL13. Changed eight times in 

ten years, Orange number six in May 1937 brought 

the Navy’s plan into line with the Joint Army-Navy 

Basic War Plan Orange. From the point of view of 

OP-20-G, this change was unique since, for the first 

time in section 7, chapter II, part I (The Strategic 

Plan), ONI was tasked to plan for the “collection, 

evaluation, and dissemination of all information of 

tor) and eventually a command, particularly after a 

two-year shore assignment at the postgraduate school 

in Annapolis. On the other hand, tours in intelligence 

or in radio intelligence, often given to people who 

had failed to obtain the assignment of choice, were 

viewed as dead-end assignments leading to poor efficiency reports upon reassignment to the fieet.68 This 

situation had a noticeably chilling effect on career 

decisions for officers and by 1936 had come to the 

attention of Admiral William H. Standley, CNO. 

Standley, one of the few senior officers of his time 

aware of the importance of radio intelligence, advised 

his personnel chief that some action must be taken to 

eliminate the stigma of such assignments.69

Nevertheless, it is clear that, with few exceptions, 

both officers and enlisted men preferred almost any 

other assignment when faced with the prospect of an 

assignment in radio intelligence.70 Although communications officers did enter the COMINT field, 

and some, such as Wenger and John R. Redman, did 

eventually become admirals, most of the officers who 

led the Navy’s COMINT effort in 1941 were either 

reservists or line officers who had willingly or unwillingly given up their chance for flag rank to serve in 

the obscurity of radio intelligence assignments.71

Moreover, until Safford’s appointment as head of 

OP-20-G in 1936, the Code and Signal Section had 

not had a permanent, full-time chief though there 

is little doubt that Safford retained control while on 

detached sea duty. In an attempt to fill the officercryptanalyst quotas spelled out in war plans at theater 

level, the Navy conducted an elementary cryptanalysis training program for reservists in several naval districts between 1934 and 1939. The numbers over the 

five-year period, however, amounted to fewer than 

119 throughout the entire Navy.72

Planning for War 

Another aspect of the perennial manpower problem concerned the capability of OP-20-G to 
28

Pearl Harbor Revisited

cryptanalyst) authorizations worldwide to a total 

of forty-three.76

The resource review acknowledged that the 

foremost operational problems facing OP-20-G 

were manpower shortages, an expanding and 

increasingly complex Japanese cryptographic environment, and the resulting cryptanalytic backlogs 

which continued to engulf the small workforce. All 

of these problems focused on research manpower. 

Having recognized the susceptibility of Japanese 

cryptography to machine exploitation, an immediate solution was to recommend the installation of 

IBM tabulating equipment in all research units as 

rapidly as funds would permit. The idea was that 

this equipment would enable fewer people to do 

more work. By equipping these units with the latest IBM equipment in peacetime and developing 

other machines to meet improvements in Japanese 

cryptography, OP-20-G believed that necessary 

cryptanalytic techniques could be developed and 

the people properly trained before hostilities began. 

Theoretically, no delays would occur after war began 

in exploiting the Japanese cryptographic systems, at 

least not through lack of equipment. 

It is both interesting and instructive to follow 

the vicissitudes of War Plan Orange (WPL13) from 

1937 to 1941 because of the bearing they may have 

had on the resource decisions made concurrently in 

DNC and OP-20-G and because they will provide a 

revealing insight into the events at Pearl Harbor on 

7 December 1941. 

A familiar and fundamental feature of 

WPL13 in 1937 was a U.S. Navy offensive into 

the western Pacific from Pearl Harbor. The initial 

objective of this operation was to either relieve 

its defenders or recapture Manila Bay. Although 

the Army thought the offensive aspects of this 

Orange plan in 1937 were “an act of madness,” 

they could not argue that Manila Bay was the best 

and possibly the only base from which to conduct 

future offensive operations in support of other 

military and economic value.” This language appears 

to have inspired extensive revisions to the Communications Service plan, Appendix 4 of which pertained to COMINT. 

Addressing issues such as wartime organization 

and subordination of communications intelligence 

within the naval establishment, the responsibility 

for dissemination of COMINT, the subordination 

of translators attached to COMINT activities, and 

U.S.–UK relationships, the DNC plan now provided for COMINT as an integrated service under 

the Chief of Naval Operations. The provisions of 

Appendix 4 also provided for dissemination of 

COMINT by the Naval Intelligence Service and, 

not surprisingly, stated that the COMINT organization “worked for the DNI under the DNC.”73

Organizationally, the 1937 plan more than reinforced the decentralized wartime operation visualized originally by DNC planners. Appendix 4 was 

revised again in 1939, and the COMINT function was somewhat streamlined by the proposal to 

drop the COMSEC function of OP-20-G into 

another appendix. Accordingly, a revised Appendix 

3 for COMSEC was completed in 1939, although 

it would be 1942 before OP-20-G was actually 

relieved of COMSEC responsibilities. 

After the war plan review of 1937, OP-20-G 

commissioned a study of its current posture.74 This 

study was probably intended to measure the current COMINT organization against the needs 

of the war plan. The results reflected a workforce 

quite inadequate to the tasks as outlined in the war 

plan. Instead of a required seven “intercept nets,” 

only five were found to exist and a total of only 

eighty-seven radiomen served the intercept and 

direction finding function of these nets.75 To be 

faced with a totally inadequate collection structure was one thing, but OP-20-G found it necessary immediately to enhance its commitment to a 

research effort. Characterizing it as the “nucleus” 

of a wartime organization, OP-20-G proposed to 

enlarge its current research manpower (i.e., purely 
 29

Planning for War

rejected a hypothesis that Japan already controlled 

the Netherlands East Indies and was poised to take 

over Singapore and the Philippines. The planners 

also considered a third alternative—that Japan had 

not yet moved southward from Formosa—since the 

central issue was at what point the United States 

would intervene. The planners rejected this alternative because they could not decide whether it would 

necessitate intervention and were not certain that 

the American people would support such preventive 

measures as early movement of the U.S. Fleet to the 

Philippines, to the East Indies, or to Singapore. 

At this stage it should be noted that Rainbow 2 

for 1939 described solely a naval war in which the 

United States had made no commitment to China. 

The plan concentrated on measures necessary to 

keep pressure on Japanese overseas lines of supply and communication. It did contain for the first 

time a specific COMINT-related task levied on the 

Naval Communications Service. The service was to 

intercept enemy communications and locate enemy 

units (using DF) and turn over the information to 

ONI for “dissemination as advisable.” Although 

many concerned voices were raised over the inherent weaknesses of WPL13 and the Pacific war features of Rainbow 2, this is the last recorded activity 

in Pacific war planning until June 1941. 

Rainbow 2 was the final Pacific-first strategic plan. It was never adopted by the Joint Board 

U.S. interests in the Far East. Here was an obvious area for future compromises. 

The Navy Basic War Plan Orange for 1938 contained three new assumptions inspired by extensive 

Army revisions to the Joint Plan, which eliminated 

all references to offensive warfare: (1) outbreak of war 

would be preceded by a period of strained relations; 

(2) Orange would attack without warning; and (3) a 

superior U.S. fleet would operate west of Hawaii. 

The eighth and final change to WPL13 was 

made in March 1939. This change reflected the initial shift in U.S. strategic thinking from the Pacific 

to events in Europe and the Atlantic Ocean, away 

from offensive operations toward a concept of 

defensive operations and readiness. At the same 

time a new planning system replaced the colors 

adopted over thirty years before with the Rainbow 

Plans described briefly as follows: 

Rainbow 1 (WPL42): Limited action in order to 

prevent a violation of the Monroe Doctrine as far south 

as 10 degrees south latitude. This plan was approved by 

the secretaries of war and navy on 14 August 1939. 

Rainbow 2: Rainbow 1 in first priority followed 

by concerted action by the United States, Great 

Britain, and France against the Fascist powers. U.S. 

forces responsible solely for the Pacific. 

Rainbow 3 (WPL44): Rainbow 1 in first priority followed by projecting American forces into the 

western Pacific. 

Rainbow 5 (WPL46): Rainbow 1 in first priority 

followed by U.S. armed forces into east Atlantic or 

Europe and Africa in concert with Great Britain and 

France. (Modified to conform to the course of the 

war in Europe during 1940 until December 1941.) 

Planning for WPL13 appears to have continued during 1939 and into early 1940. Attempting to 

add realism, planners in September 1939 assumed 

that Japan would dominate the Asian coast and 

adjacent waters as far south as Indochina. They 

In 1940 the fate of England and 

control of the Atlantic Ocean 

were the most vital planning 

issues in American policy.
30

Pearl Harbor Revisited

5. Undertaking of “progressive” mobilization including a draft and other measures 

to accelerate production of war material and 

training of personnel. 

6. Beginning of preparations for the 

“almost inevitable conflict” with totalitarian 

powers.77

Although planning for war with Japan was not 

extinct, the end was now near. On 25 September 

1940, a memorandum prepared by Army planners 

for their boss, Major General George V. Strong, 

examined U.S. prospects in the event of a British 

defeat in the Atlantic in the context of the American commitment in the Pacific (i.e., Rainbow 3 vs. 

Rainbow 4) and concluded that they were incompatible policies. Army planners went one step further and warned against a more active policy of pressure toward Japan. They recommended rapid U.S. 

rearmament, aid to Great Britain, refraining from 

antagonizing Japan, remaining on defensive in the 

Pacific, and finally, moving to ensure the security of 

the western Atlantic. 

In a similar study two months later, Navy war 

planners under Captain Richmond K. Turner discovered that realistic Pacific operations under Rainbow 3 would be impossible if the naval detachment 

required under Rainbow 4 were transferred to the 

Atlantic. With the forces available, they reported, 

the U.S. Navy could operate in only one theater. This 

discovery led Admiral Stark to write his famous 

“Plan Dog” memorandum to Secretary of the Navy 

Frank Knox on 12 November 1940. The ideas contained in his memorandum had not changed significantly between June and November although they 

did reflect some of General Strong’s thoughts from 

September. His conclusion, however, was remarkable: the United States might “do little more in the 

Pacific than remain on a strict defensive.” Clearly, 

the first U.S. priority was to the British war effort 

and to prevent the war in Europe from spreading to 

the Western Hemisphere. Still, it is startling to see 

or published. Beginning in early 1940, the entire 

focus of American strategy changed following 

Germany’s easy victories in Norway and Denmark. The shift in focus was signaled by a letter 

from the Joint Planning Committee to the Joint 

Board on 9 April 1940, recommending that planning begin immediately under Rainbow 5, leaving Rainbows 3 and 4 in skeletal form. With this 

letter, Pacific-first strategic thought and planning was virtually at an end. The fall of France in 

June 1940 and the subsequent Battle of Britain 

raised serious questions about the security of the 

United States itself, whether or not the British 

Isles would fall as France had, and the fate of the 

British Navy. Suddenly, the fate of England and 

control of the Atlantic Ocean were the most vital 

planning issues in American policy. 

The brief but interesting evolution of Rainbow 

5 from being one among equals to the preeminent 

U.S. war plan is also instructive. It not only involves 

the final stages of the other four plans, but its 

details, too, lend insight to the events of December 

1941. 

After the fall of France in June 1940, General George C. Marshall, Army Chief of Staff, and 

Admiral Harold R. Stark, Chief of Naval Operations, submitted to President Roosevelt a draft entitled “Bases for Immediate Decisions Concerning 

National Defense.” As amended after the president’s 

views were obtained, it became on 27 June 1940 a 

plan for national defense. Its six provisions were as 

follows: 

1. Assumption of a defensive posture by the 

United States. 

2. Provision of support for the British Commonwealth and China. 

3. Implementation of Rainbow 4 actions for 

defense of the hemisphere. 

4. Cooperation with certain South American countries. 
 31

Planning for War

able to OP-20-G were also assigned to Washington, 

where cryptanalytic research was the primary mission. In earlier discussions of Japanese naval systems, 

it was noted that the year before only from two to 

five people could be spared to work on JN-25. There 

is, moreover, ample reason to believe that emphasis had not changed materially by December 1941. 

Given the new Germany-first policy, how interested 

were naval decision makers in OP-20-G, War Plans, 

and Operations in the Pacific-related intelligence 

being pumped out by Pearl Harbor, Corregidor, and 

OP-20-G? These units were individually and collectively flooding the desks and in-baskets of Navy 

officials in Washington with alarming reports of 

Japanese war preparations, some of which must have 

read like cribs from a Rainbow 2 planner’s wastebasket. Yet, as became painfully clear throughout 1941, 

only readable messages were bankable, and only 

Japanese diplomatic messages were being deposited. 

On 11 June 1941, Admiral Stark formally canceled 

WPL13.79

the Chief of Naval Operations, in the fall of 1940, 

advocating a policy of avoiding even a limited war 

with Japan after over thirty years of planning for an 

unlimited offensive war. The only concession Stark 

would make was to leave the U.S. fleet at Pearl Harbor because of the U.S. diplomatic commitments in 

the Far East. His firmness in this purpose was to 

cost Admiral James O. Richardson, Commander in 

Chief, U.S. Fleet, his job.78 

Cause-and-effect relationships are often difficult to establish, particularly in resource decisions in 

Naval Communications and OP-20-G. Frequently 

one must work backwards from the end result. Using 

this method, it is clear that manpower resources in 

OP-20-G were adversely affected by the CNO shift 

in policy and planning from Pacific-first to Germany-first. Chart C graphically displays the fact that 

in December 1941 over 60 percent of all COMINT 

manpower had been concentrated in Washington 

where the only current mission was Japanese diplomatic and Atlantic DF. It is also clear in Chart 

C that two thirds of the officer cryptanalysts availChart C 

Distribution of Navy COMINT Personnel, December 1941

Category

Atlantic: 

Navy Dept.

Pacific: Pearl 

Harbor

Asiatic: 

Corregidor In Transit Total

Officers 53 12 9 6 80

Cryptoclerks 157 18 19 20 214

Subtotal 210 (47%) 30 (16%) 28 (26%) 26 294 (40%)

Intercept 

Stations/

DF Control 178 72 42 — 292

Outlying DF 

Stations 60 84 8 — 152

Subtotal 238 (53%) 156 (84%) 50 (64%) — 444 (60%)

Totals 448 (61%) 186 (25%) 78 (10%) 26 (3%) 738 (100%)
32

Pearl Harbor Revisited

ONI would provide the necessary information, and 

the board would prepare a plan aided by the staff of 

the War College. In fact, the College did most of 

the work. 

Before World War I, students at the War College raised the issue of planning for wars without 

guidance from the political establishment. They 

felt that trying to plan without the answers to such 

questions as, What are the intentions of the United 

States in China, Japan, the Monroe Doctrine? was 

an exercise in futility. Their questions found willing 

listeners on the board, at ONI, and even in the secretary of the navy. The Navy proposed various remedies to both the legislative and executive branches 

based on the premise that “plans not based on the 

interrelation of the enemy’s and our own motives are 

of little value”—but to no avail. 

In March 1912 the General Board, under its 

chairman, Admiral George Dewey, broke with the 

College over its objections to the vague and narrow terms of a war plan request. In his decision 

Admiral Dewey dictated that military men should 

limit their curiosity to “purely military questions. A plan can be prepared for a specific purpose without reference to any matter not bearing 

directly on the purpose in view. . . . A commander 

in chief should, therefore, rarely be influenced by 

ulterior motives.” 

The effects of this decision were apparent at 

the War College as recently as the 1960s, when 

the curriculum was described as “not focusing on 

the specific political consequences past, present, 

or future of military actions.” In the context of 

the events of 1940 and 1941 both in the Pacific 

and in Washington, the effects produced planners 

whose perceptions of Japanese naval capabilities 

and national intentions may have been seriously flawed by war planning doctrines which ruled 

out enemy intentions altogether as unreliable and 

subject to rapid change. Under these circumstances, COMINT producers who provided strategic 

War Games 

Closely aligned with the planning function are 

war games. Games are invaluable for testing 

all elements of a plan. Their scenarios are a mixture 

of capabilities and objectives which may be deliberately arranged to test a specific plan as a whole or a 

single element within a plan. 

War games were introduced into the U.S. Navy 

in the late nineteenth century by Lieutenant William McCarty Little, a member of the staff of the 

Naval War College. As a result, early in the twentieth century, the War College virtually backed into 

a war-planning relationship with the Navy’s General Board. The board was to designate a country 

for which a Naval War Portfolio was to be prepared; 

Captain Thomas H. Dyer, USN, was 

assigned cryptanalytic responsibility 

for the Japanese Navy’s Flag 

Officers Code.
 33

National Emergency

equipment he was to train the clerks himself. Typically, he was instructed to provide “material assistance to Washington” as soon as possible.83 Again, 

the manpower problem imposed severe limitations 

on potential U.S. successes against the Japanese 

codes. 

National Emergency 

I

n September 1939 all restrictions on increasing 

personnel and installations were removed when 

the United States declared a national emergency. A 

few weeks later most of the earlier “Neutrality Legislation” was overturned. Nevertheless, manpower 

shortages continued to plague the Navy COMINT 

program, particularly in the Pacific, well into 

1941.84 In June 1940 Admiral Claude C. Bloch, 

COM-14, requested more manpower (twenty-one 

billets) to expand his COMINT operation.85 In his 

favorable endorsement to the Bureau of Navigation, which handled all Navy personnel matters, 

Admiral Harold R. Stark, CNO, made the following observation: “The main obstacle to expansion 

[of communications intelligence activities] is not 

the matter of increased allowances but finding suitable personnel to fill existing allowances. Many of 

the reserve personnel in DNC’s mobilization slate 

are reluctant to leave their civilian occupations 

prior to full mobilization, and COM-14 has been 

unable to find any suitable volunteers.”86 Later in 

1940 Safford complained, “We are allowed 75 and 

actually have 55.”87

Compounding the continuing manpower shortage by adding inordinate delays to the system of 

exchanging crypt recoveries between Washington 

and the field was a severe and perennial communications problem which affected all Navy COMINT 

initiatives, particularly those against the Japanese. 

Therefore, even before they broke down completely when war began, the primitive methods of U.S. 

Navy communications and the centralization of 

warning beginning in September 1941 that Japan 

was preparing for war should not have been surprised that their warnings were ignored until the 

eleventh hour.80

Pacific Buildup 

After an abortive attempt at across-the-board 

modernization in the early Depression years, 

virtually all OP-20-G’s attempts to increase manpower and improve equipment during the 1930s 

were directed toward the Pacific basin where the 

Japanese threat was seen as paramount. In 1937 OP20-G opened the long anticipated major research 

unit in Hawaii with the task of supporting Washington’s efforts.81 Lieutenant Commander Thomas 

Dyer was detached from fleet duty and assigned to 

COM-14 as a cryptanalyst. His duties were to establish a decrypting unit and undertake research work on 

“M1 Orange Naval Cryptographic System.”82 Message files for 1935, 1936, and 1937 were supplied by 

OP-20-G and sent to Dyer via the USS Chaumont. 

COM-16 sent copies of all traffic, including messages intercepted by Stations A, B, and C to Dyer, 

and Washington mentioned that IBM equipment 

would be forthcoming at once. At the outset, when 

not occupied by other duties assigned by COM-14, 

Dyer, who was not a Japanese linguist, singlehandedly 

attempted to recover all keys as they appeared. He 

naturally forwarded all solutions to Washington for 

translation. In 1939, when the M1 system had been 

exhausted, Dyer was assigned cryptanalytic responsibility for the Japanese Navy’s Flag Officers Code. 

Lieutenant Joseph Richardson appeared the following July as language officer. 

The IBM equipment promised in 1937 was 

not immediately forthcoming. In February 1938, 

however, OP-20-G notified Dyer that he was to 

receive IBM tabulating machinery and two clerks. 

Taking a little of the glow from the moment, Dyer 

was also told that after becoming familiar with the 
34

Pearl Harbor Revisited

before COMINT would become a useful instrument 

during wartime was clear.90

Circumstances Favor 

Diplomatic Targets 

Electrical communications within the continental United States were only slightly better than those overseas, and only in rare cases 

did they serve to speed the flow of information 

to Washington from abroad. In August 1940 the 

Navy had five sites with diplomatic targets which 

were all linked directly (or indirectly through 

Army circuits) to Washington via radio and landline communications. These sites were Winter 

Harbor, Maine; Amagansett, New York; Cheltenham, Maryland; Jupiter, Florida; and Bainbridge 

Island, Washington.91 Radio communications 

with Hawaii consisted of single-channel morse 

links between Washington, San Francisco, and 

Pearl Harbor. Landline communications consisted of the relatively higher capacity government 

and commercial teletype circuits owned or used by 

each military department. The Army owned 401 

mainly east-west circuits with 1,003 machines. 

The Navy owned forty-four mainly north-south 

circuits on the coasts, the first of which was not 

established until 1941. These circuits served 

naval activities at Washington, Norfolk, Philadelphia, New York, New London, Boston, and 

Portsmouth, New Hampshire.92 In March 1941 a 

commercial teletype line between Winter Harbor, 

Amagansett, and Bainbridge Island was inaugurated.93 Despite the availability of teletype circuits 

from the West Coast to Washington, DC, there is 

no indication that packages containing Japanese 

intercept from the Pacific which arrived by either 

air or sea throughout the 1930s were opened 

until they reached Washington through normal 

overland channels. In 1939 triple-wrapped packages from Guam containing the daily bundles of 

messages were received by the “Courier Station” 

cryptanalytic functions proved to be major liabilities 

which prevented realization of the ultimate organizational goal, support for the fleet commander. The 

addition of officer-cryptanalysts to Station C and 

to the centers in Hawaii was not successful in this 

regard because code and key system recoveries from 

Washington, when available, were delayed by the 

surface transportation system of commercial vessels 

and railroads. As a result, completely current English texts were probably a rarity in the Philippines 

before the war and were never seen in Hawaii until 

March 1942. 

The individual radio intelligence officer-analysts 

assigned to the Asiatic Fleet and the 14th and 16th 

Naval Districts between 1930 and 1940 were, by 

today’s standards, almost completely isolated from 

Washington. Communications between Washington 

and its far-flung resources in the Pacific continued 

to be primitive until long after 7 December 1941. 

Messages and intercept logs, reports and professional 

correspondence, if classified, were painstakingly enciphered by the radio intelligence officer himself using 

special equipment and instructions.88 If transmitted 

as messages on manual morse circuits or landlines, 

they were delivered to the communications center 

where they were again enciphered. Material such as 

traffic logs and routine Japanese messages, however, 

were always sent home by mail. A package was usually forwarded once a week from Hawaii, Guam, and 

the Philippines. It would be put aboard a commercial 

ship or a station-keeping vessel like the USS Chaumont, which traveled the Pacific from California to 

China. After 1935 a small amount of mail could be 

sent via the Pan American Airways “Clipper” (large 

passenger seaplanes) using a small strongbox built 

into the hull specifically for that purpose. Keys to 

open the strongbox were held by authorized officers at appropriate points along the route. The code 

known as RIP 30 was developed specifically for air 

mail letters.89 The introduction of air mail service in 

1935 reduced travel time from weeks to days, but the 

need for major improvements in communications 
 35

Cooperation with Allies 

available in Washington prior to Pearl Harbor was 

Japanese diplomatic traffic.95

Three events involving the sites at Cavite and 

Heeia, Hawaii, from 1938 to 1941 will strikingly 

illustrate how truly primitive were the communications that served the Navy’s COMINT function 

overseas. In September 1938 Lieutenant Jack S. Holtwick, then the CINCAF Radio Intelligence Officer, 

complained to OP-20-G about the lack of electrical 

communications between the unit at Cavite and the 

flagship. He said that “it now takes days to obtain 

COMINT information needed to prepare a daily status report.” In 1940 Hawaii commented on tracking 

Japanese naval vessels during annual maneuvers, stating that “the only helpful direction finding came from 

the Philippine unit by Clipper mail!”96 Finally, on 5 

January 1940 Admiral Stark, CNO, requested the 

Bureau of Engineering to connect the site at Heeia 

to an Army cable which then terminated at Kailua, 

eight miles away. Stark also requested the engineers 

to arrange for an intercom between the communications intelligence unit at Pearl Harbor, the Lualualei direction finding site, and Heeia, also by Army 

cable, using “other than teletype instruments.” These 

arrangements were meant to replace the public party 

line telephone service. By 7 December 1941 the work 

had not been done, and with the loss of telephone 

service in the attack, there were no communications 

between Heeia and Pearl Harbor (about thirty road 

miles) except by vehicle.97

Cooperation with Allies 

Until August 1941, efforts to recover JN-25B 

code values were restricted to the British force 

at the Far East Combined Bureau (FECB), Singapore, and four U.S. officer-linguists at Corregidor, 

working in close collaboration with the British. In 

August 1941 OP-20-G, Washington, began to help 

with JN-25B code recovery but was hampered by 

lack of linguists familiar with Japanese naval termiin San Francisco, where they were opened and 

placed in registered mail. The inner package was 

marked, “To be opened only by OP-20-G.” In the 

Philippines during the 1930s, intercept material 

was mailed to Washington in a similar manner.94

Information Gaps 

By 1941 the mission constraints in Corregidor 

and Hawaii improved by their limited cryptanalytic capabilities, and the pervasive shortages 

of all types of manpower in Washington, contributed to a growing sense of alarm in OP-20-G. As 

the year progressed, certain daily summaries produced by Hawaii and Corregidor, particularly those 

which sounded warning signals, were no doubt 

marked for electrical forwarding. But the capacity 

of manual morse circuits and the inherently slowmoving manual decryption features of Navy communications between the mainland and overseas 

stations were contributing to a serious information 

gap between Washington and the fleet-supporting 

field activities. 

In July 1941 the nucleus of what would become 

OP-20-GC (communications) in early 1942 was 

formed using personnel from OP-20-GY (cryptanalysis). The objective of this new element was to 

encrypt, decrypt, route, deliver, and file COMINT 

dispatches between OP-20-G and the outside 

world. In theory this organization was intended to 

be much faster and much more secure than the Navy 

Code Room. The whole affair, unfortunately, was 

undertaken on an incredibly small scale. By December 1941 it consisted of two reserve ensigns, neither 

of whom were trained or experienced in communications. Under these circumstances—which seem 

doomed to fail—it is little wonder that OP-20-G’s 

customers were attracted to readable diplomatic 

messages collected by mainland sites (see Chart A). 

It is also little wonder that, at this time, the principal 

source of crisis-related communications intelligence 
36

Pearl Harbor Revisited

tance by Admiral James O. Richardson, Commander 

in Chief, U.S. Fleet. Richardson and most Navy officials who opposed the move thought a fleet anchored 

in Pearl Harbor would be unnecessarily exposed to 

Japanese naval strength. President Roosevelt, however, considered the move as a necessary countermeasure to growing Japanese bellicosity. Throughout 

1940 Richardson bitterly voiced his objections to 

relocating his headquarters to Pearl Harbor because 

it challenged the soundness of U.S. policy in the 

Pacific. He claimed that a Pacific naval offensive—

the heart of the Navy’s War Plan Orange—was sure 

to fail because the United States did not have the 

capability to support an offensive west of Hawaii. He 

also noted a factor not considered by the war planners: the United States was now vulnerable to attack 

in the Atlantic and the Caribbean. In January 1941 

Roosevelt ordered Richardson relieved. His replacement was Admiral Husband E. Kimmel, who at the 

same time was designated Commander in Chief, U.S. 

Pacific Fleet (CINCPACFLT). 

During this period of great internal upheaval in 

the Pacific Fleet, two relatively insignificant events 

occurred which actually marked the beginning of a 

close COMINT relationship between that fleet and 

OP-20-G. On 7 December 1940 exactly one year to 

the day before Japan attacked Pearl Harbor, Lieutenant Commander Edwin T. Layton, a Japanese 

linguist with past experience in OP-20-G, became 

the Fleet Intelligence Officer, and a few months 

later, Commander Joseph J. Rochefort, who was the 

only man in the Navy who was both a cryptanalyst 

and a Japanese linguist, became OIC of the 14th 

Naval District’s Radio Intelligence research effort. 

Layton and Rochefort were old friends from sea 

duty and from language training in Japan. Rochefort 

was assigned to the fleet when he was transferred 

to COM-14. Both knew of the OP-20-G operation, having served under Safford in the 1930s. Layton served only briefly, but Rochefort had received 

extensive training as a cryptanalyst. They quickly 

established a close working relationship, and the 

nology and usage and by the slow communications 

available at the time. The only current JN-25 messages read by U.S. analysts on Corregidor during 

this period were few and invariably ship movement 

reports: arrivals and departures, together with some 

fragmentary schedules. In view of the full collaboration and exchange with FECB, Singapore, there is 

no reason to believe that the British exceeded the 

U.S. accomplishments. 

The Move to Hawaii 

On 7 May 1940 the U.S. fleet moved its headquarters from San Pedro, California, to Pearl 

Harbor. The move was undertaken with great relucLieutenant Commander Edwin T. Layton, 

a Japanese linguist, became Fleet 

Intelligence Officer.
 37

Support to the Pacific Fleet 

Collectively they revealed a wealth of information 

concerning Japanese naval activities, particularly 

those under way in the Mandates, on the islands of 

Hainan and Taiwan, and along the Chinese coast 

(see Appendix C). 

In many respects COM-14’s efforts and 

achievements in 1941 were similar to what had 

been accomplished at Station C with traffic analysis against the Japanese Imperial Fleet maneuvers in 

the 1930s. The COM-14 daily summaries clearly 

showed that Lieutenant Thomas A. Huckins and 

Lieutenant John A. Williams, who headed the traffic analysis unit, had solved both the strategic and 

tactical Japanese naval communications structures. 

They understood the callsign generation system 

and were able to quickly reestablish order of battle 

data after routine callsign changes. This insight permitted unit identifications to the squadron level in 

ground-based air and destroyer units. It also allowed 

liaison would soon prove immensely beneficial to 

the U.S. Pacific Fleet.98

Both Rochefort and Layton were called upon 

regularly to brief Kimmel on what COMINT 

revealed about the Japanese Navy. In one instance 

Layton’s analysis of callsign and address usage, 

which he had undertaken during 1941 at Rochefort’s request, was sent to Washington on the order 

of Admiral Kimmel. His conclusion that the Japanese had begun a military buildup in the Mandate 

Islands (Marianas, Carolines, and Marshalls) was a 

development that had gone unnoticed by COMINT 

analysts in Washington. Unexpectedly, rather than 

foster good relationships between Pearl Harbor and 

Washington, this episode caused considerable ill 

feeling toward Layton and Rochefort. It may also 

mark the beginning of an unhealthy intramural OP20-G rivalry between the Washington and Hawaiian centers over the issues of COMINT reporting 

responsibilities and Japanese intentions which persisted well into 1942. 

Support to the Pacific Fleet 

From July 1941 onward, the COMINT research 

unit in Hawaii under Rochefort prepared 

daily COMINT summaries for Admiral Kimmel 

(see Appendix C). They were based on analysis of 

Heeia collection and to some extent on technical and intelligence information from Corregidor. 

Hawaii’s analytic contributions to the summaries 

were based on traffic analysis of message externals and direction finding results since the Flag 

Officer’s Code could not be read, and they had no 

capability against the Japanese Fleet Operational 

Code ( JN-25). These summaries were characterized by Layton after the war as containing “no hard 

intelligence.” This is a harsh judgment. Individually, though it is true that they contained no Japanese message texts, the summaries constituted the 

substance of Layton’s daily reports to CINCPAC. 

Commander Joseph J. Rochefort, the only man 

in the Navy who was both a cryptanalyst and a 

Japanese linguist
38

Pearl Harbor Revisited

Support to Asiatic Fleet 

Corregidor too was very active in following Japanese fleet naval and air movements throughout 1941, producing reports for much of the year 

in technical channels, which included the CINCAF 

radio intelligence officer and the Hawaiian Research 

Center. It was not until October 1941 that Station 

C’s technical reports began to appear as daily intelligence summaries. Rochefort’s daily reports often 

contained information derived from reports from 

Corregidor.102 In late November, because of its scope 

and the station’s central location, COM-16’s perspective was judged to be superior to Hawaii’s and to 

the fragmentary and often conflicting reports from 

other sources such as attachés in Shanghai, Chungking, and Tokyo. On 24 November 1941 Admiral 

Stark, CNO, ordered Admiral Hart, CINCAF, to 

receive, evaluate, and combine all reports and conclusions, including those from COM-14, reporting directly to CNO with information to Admiral 

Kimmel, CINCPAC.103 In the two weeks of peace 

remaining before Pearl Harbor, this order had little 

or no effect on events.104

Japanese Strategy 

While the United States attempted to maintain 

a level of strategic equality with Japan in the 

Pacific by offsetting losses of capital ships sent to 

the Atlantic with a buildup of long-range air power, 

the Japanese government formulated plans for war 

in the Pacific. The Japanese war plan for the Western Pacific campaigns began to unfold well before 10 

November 1941 when General Count Hisaichi Terauchi, commanding the Southern Army, and Admiral Isoroku Yamamoto, commanding the Combined 

Fleet, formally concluded a “Central Agreement” 

which outlined an ambitious scheme of Japanese 

conquests. 105 According to the agreement, the first 

operational stage was divided into three phases: (1) 

attacks on the Philippines, Malaya, Borneo, Celeidentifications to the individual ship level in battleships, cruisers, and carriers. The capability to exploit 

these features of Japanese Navy communications 

lasted until about three weeks prior to the attack 

on Pearl Harbor when callup and addressing procedures changed abruptly.99 Throughout the period 

they were also able to use their direction finding 

capability to produce unique information as well as 

to support evidence from traffic analysis. The traffic 

analysis unit was able to identify the Japanese Navy 

mainline shore establishment from Imperial and 

Combined Fleet Headquarters to principal line and 

staff subordinates within each of the fleets in both 

home and deployed locations. Based on the content 

of their daily summaries, it is conceivable that communications being intercepted by Hawaii (Heeia) in 

1941 encompassed the entire Japanese Navy communications system ranging from Japan to South 

China, to the Mandate Islands, and to the connecting ocean area.100 Intercepted messages were mailed 

to Washington for exploitation of their texts. 

Japanese Intentions Revealed 

As early as July 1941, traffic intelligence reports 

(i.e., reports founded on traffic analysis) prepared for Admiral Claude C. Bloch, COM-14, 

and Admiral Kimmel, CINCPACFLT, reflected 

Japanese air and naval concentrations “awaiting the 

assumed Southern operations.” In fact, from July 

until 6 December, summaries from Hawaii made 

frequent allusions to the “formation of Task Forces” 

and forthcoming “hostile actions” and called attention to similarities between current activities and 

those which preceded earlier Japanese naval and 

military campaigns in South China and Indochina.101 Bearing in mind that Hawaii could not read 

the message texts, the accuracy of these reports was 

truly remarkable. 
 39

Japanese Navy

Inoue (CINC4), was to seize Rabaul, Wake, Guam, 

and Makin using a reinforced infantry regiment of 

5,000 men (the South Seas Detachment). Force 3 

consisted of fighting units from the 2nd and 3rd 

Fleets, the 11th Air Fleet, and the China Area Fleet, 

under Vice Admiral Nobutake Kondo (CINC2); 

carriers from CarDivs 3 and 4; light and heavy 

cruisers and destroyers from the entire Combined 

Fleet; as well as hundreds of troop transports, supply 

vessels, escort vessels, and oilers, and the Southern 

Army under General Count Hisaichi Terauchi. It 

was to attack the Philippines, Thailand, and Malaya 

(the Kra Peninsula and Singapore). It was to follow up this with attacks on the Netherlands East 

Indies and Burma.108 In addition to providing escort 

and cover for the Malay-Thailand invasion, the role 

of the 2nd Fleet included being Force 4, a “Distant 

bes, Timor, Sumatra, and Rabaul (also Guam, Wake, 

and Makin); (2) capture of Java and the invasion of 

southern Burma; and (3) conquest of all Burma. The 

Japanese then envisioned pacification of the area, 

the creation of a Greater East Asia Co-Prosperity 

Sphere, and probably a defensive struggle against 

the United States to maintain their hold on the 

region. A second operational stage also covered by 

the agreement was to “occupy or destroy as speedily as operational conditions permit,” eastern New 

Guinea, New Britain, Fiji, Samoa, the Aleutians, 

Midway, and strategic points in the Australian area. 

According to historian John B. Lundstrom, this is as 

far as Japanese planning went.106

Japanese Navy 

The portion of the Japanese Navy that was to 

execute an attack on Pearl Harbor and provide 

cover and escort for the remainder of these operations 

had been preparing for its various roles for several 

weeks. It consisted of 10 battleships (BB), 6 cruisers, 112 destroyers (DD), and 65 submarines (SS). In 

addition, Japan had large numbers of auxiliary vessels, tenders, minesweepers, and escorts. The fleet was 

organized into nine naval stations in the homeland 

area, the China Area Fleet, and the Combined Fleet. 

The Combined Fleet, which consisted of five mobile 

fleets (1st, 2nd, 6th, 1st Air Fleet, 11th Air Fleet), and 

three localized fleets (3rd, 4th, and 5th) was destined 

to carry the burden of the southern strategy as well as 

to conduct the strike on Pearl Harbor.107

In the opening campaigns of the first phase, the 

Combined Fleet was divided into four task forces. 

Force 1 was a carrier strike force consisting of all 

six fleet carriers, two battleships, and three cruisers 

under Admiral Chuichi Nagumo. It was to conduct 

a separate attack on Pearl Harbor. Force 2, the South 

Seas Force (4th Fleet), extensively reinforced with 

land-based air units from Japan and submarines 

from the 6th Fleet, under Vice Admiral Shigeyoshi 

Admiral Isoroku Yamamoto, Commander-inChief, Combined Japanese Fleet
40

Pearl Harbor Revisited

Because JN-25 messages as well as naval messages in other cryptsystems were largely unreadable, 

throughout the last few months of 1941 the messages were usually exploited for what their externals 

revealed (e.g., addresses, callsigns, association with 

others) and sent to Washington, where concentrated work on code and key recoveries was conducted. With some exceptions, the callsign change 

on 1 November seriously complicated the work 

of traffic analysis by introducing at the same time 

new procedures for addressing messages in which 

individual units were no longer called or addressed 

openly in the externals. (See Appendix C, note by 

COM-14 on 6 November.) Accordingly, when this 

practice was recognized, the record suggests that 

both COM-14 and COM-16, while still able to follow developments in the southern areas, had failed 

to establish continuity on the 1st Air Fleet callsign, 

which was noted and first identified on 3 November 

by COM-14 (see Appendix C). Moreover, COM14 apparently neglected to review October traffic 

in which this fleet was also active and to make the 

correct associations regarding 1st Air Fleet organization. Regrettably, the record of Appendix A and 

Appendix C also suggests that between 1 and 17 

November only message traffic that could be associated with pre-1 November southern area activity 

was examined even for its externals. The residue, 

including traffic pertaining to 1st Air Fleet activity, 

was apparently sent to Washington, which had no 

traffic analysis capability at this time and was concerned with only the cryptographic technicalities.111

Between early September and 4 December 

1941, U.S. COMINT units at Pearl Harbor, Corregidor, and Guam intercepted and forwarded to 

Washington many thousands (26,581) of Japanese 

naval messages in the fleet general-purpose system ( JN-25), a fleet minor-purpose system, a merchant vessel-navy liaison system, a merchant vesselnavy five-letter cipher, and a naval attaché cipher. 

Hawaii had no capability against JN-25, however, 

and because shortages of manpower in WashingCover Force” for the forces invading the Philippines. 

Command of naval forces directly covering invasion 

of the Philippines was given to CINC 3rd Fleet, 

Vice Admiral Sankichi Takahashi.109

In addition to the vessels and their escorts, the 

Strike Force consisted of three submarines, I19, 

I21, and I23 on Ship Lane Patrol, 2DDs as Midway Neutralization Unit (presumably the same unit 

cited in the message of 16 November 1941, shown 

in Appendix A), and a train of eight tankers and 

supply ships.110

Details of the formation, training, and assembling of each of these Japanese naval elements 

(except for the Pearl Harbor Attack Force), as well 

as the supporting Japanese air elements involved 

in the Southern operations, were reported by the 

COMINT centers in Hawaii and Corregidor. Specifically, they observed Japanese air and naval forces 

gathering in the vicinity of Takao and Keeling on 

Formosa and Mako in the Pescadores, a group of 

islands between Formosa and China. They also noted Japanese assault forces gathering on Amami O 

Shima north of Okinawa and in the Palau Islands 

in the Mandates. Air support for the Philippine 

assaults was also seen assembling in the Palaus and 

on Formosa. 

Because shortages of manpower 

in Washington precluded 

code and cipher exploitation, 

none of these systems were read 

on a current basis ...
 41

Japanese Navy

By 12 November 1941, the carriers in the 

Strike Force had completed necessary repairs and 

had returned to their respective home ports or navy 

yards. Virtually all preparations for the Pearl Harbor 

assault had been completed. Two exceptions were 

the final deployment of the Strike Force to its point 

of departure, Hitokappu Bay in the Kuriles,122 and 

completion of modifications to some oilers which 

were probably those involved in refueling the Strike 

Force on its return trip.123 On 11 November 1941, 

however, CINC 1st Air Fleet issued a routine movement message containing a plan for anchoring at an 

unspecified future date CarDivs 1, 2, and 5 and several escort units and Maru (commercial) vessels in 

Saeki Bay in the Inland Sea.124 There was no message confirming the fleet’s arrival and, while it is 

entirely possible that not all elements of the Strike 

Force deployed to the Kuriles, the routine-appearing 

message, augmented on 1 December 1941 by deceptive radio broadcasts from Tokyo,125 probably represented an attempt on the part of the Japanese to 

deceive U.S. monitors. Other Japanese naval messages now available clearly indicated that the Strike 

Force would be at sea by that date. 

On 9 November 1941, the commander of 

Destroyer Squadron 1, a Strike Force unit (Chart 

A), while coordinating his activities with the Naval 

General Staff (NGS) Tokyo, sent a message which 

revealed that on 15 November 1941 fleet carrier Hiryu 

of CarDiv 2 would be conducting a refueling drill off 

Ariake Bay while towing the Kokuyoo Maru.126 In 

addition, examination of movement reports between 

17 and 20 November 1941 reveals that the Strike 

Force flagship at that time was the battleship Hiei and 

that it was located at Hitokappu Bay (approximately 

45-00N 147-40E).127 Finally, on 19 November 1941 

CINC Combined Fleet announced to all flagships a 

communication exercise on 22–23 November 1941, 

which excluded “the forces presently en route to the 

standby location.”128 Collectively, although not definitively, these messages strongly suggest that since 15 

November 1941, instead of anchoring in Saeki Bay, 

ton precluded both code and cipher exploitation, 

none of these systems were read on a current basis 

even though Corregidor may have been nominally 

responsible for their exploitation.112

Had these messages been exploitable at the 

time, their stunning contents would have revealed 

the missing carriers and the identity of other major 

elements of the Strike Force.113 Not only did the surviving messages (which were finally decrypted and 

translated in 1945 and 1946) provide the identity 

of the 1st Air Fleet’s Strike Force, but they revealed 

the Strike Force’s objective through analysis of its 

exercise activities and its movements prior to 26 

November 1941 (see Appendix A).114

The method of attack and objective of the Japanese Strike Force were revealed in messages intercepted between 21 October115 and 4 November 

1941.116 On 21 October, Carrier Divisions 1, 2, and 

5 began a series of exercises and training maneuvers 

that involved specially modified torpedoes.117 These 

exercises, which probably ended on 6 November 

1941, when CarDivs 1 and 2 were “to fire (torpedoes) against anchored capital ships” in Saeki Bay, 

amply demonstrated that the Strike Force had a 

naval objective.118 Furthermore, the extraordinary 

measures taken by the Combined Fleet to insure 

adequate fuel supplies for the Strike Farce demonstrated that the naval objective was at a distant point 

far removed from shore-based fuel and even beyond 

the normal Japanese resupply capability. Between 4 

October and 1 December 1941, the Chief of Staff 

Combined Fleet, CINC 1st Air Fleet (commander 

Strike Force), units of the Strike Force, and many 

Japanese navy yards exchanged messages which 

revealed that three of the carriers (Akagi, Soryu, and 

Hiryu) would carry fuel oil as deck cargo and in 

spare fuel tanks,119 that additional oilers had been 

requisitioned into the Strike Force and modified for 

refueling at sea,120 and that carriers and their escorts 

would conduct extensive practice of refueling while 

under way.121
42

Pearl Harbor Revisited

14’s speculation regarding Japanese carriers in the 

Mandates (i.e., CarDiv 3, Ryujo, and one Maru vessel). In a curious and unexplained reversal, however, 

COM-16 stated that COM-14’s report could not be 

confirmed. It was also in this confusing context that 

COM-16 reassuringly and incorrectly reported that 

as of 26 November 1941 “all First and Second Fleet 

carriers are still in [the] Sasebo-Kure area.” The two 

summaries from Hawaii and Corregidor on 26 and 

27 November 1941, respectively, were thus unique 

not because of their imperfections but because they 

clearly showed Washington the current military 

situation in the Pacific as perceived by radio intelligence centers in the Pacific and Asiatic Fleets. It was 

entirely possible, as Layton later claimed, that the 

OPNAV warning message of 29 November 1941 

was a direct result of the impact of these summaries 

on the Chief of Naval Operations. In view of the evidence, however, an even more likely possibility was 

that all the OPNAV warning messages of November were stimulated by COMINT: Japan’s hostile 

intentions from the diplomatic messages and the 

likely targets from the daily COM-14 and COM16 summaries that had inexorably found their way 

into the consciousness of official Washington.130

Diplomatic Messages 

No review of the Navy’s COMINT contribution to U.S. knowledge of Japanese pre-Pearl 

Harbor intentions would be complete without citing the benefits U.S. officials derived from the messages exchanged by Japanese diplomats in Washington and Tokyo. Although the credit for initial 

U.S. success against Japanese diplomatic machine 

systems must go to Army cryptanalysts, the Navy 

did play a significant role in providing collection 

and, after October 1940, in devoting the bulk of its 

cryptanalytic and linguistic resources to the exploitation effort. Unfortunately, as Safford had foreseen, 

the small Navy cryptanalysis effort in Washington 

was almost overwhelmed by the volume of messages 

major elements of the Strike Force had in fact been 

at sea probably moving to the high north latitudes of 

the Kuriles or, in the case of late departures, toward 

the east on the 30° line. 

While the above information was not available 

at the time, daily traffic intelligence reports based 

on traffic analysis of communications of the Japanese Second, Third, and Fourth Fleets concerning 

events in the western and west-central Pacific areas 

were produced by both Hawaii and the Philippines. 

These reports were mailed to Washington where, 

after about two weeks en route, they formed the 

basis of biweekly OP-20-G summaries prepared for 

ONI.129 Although the material was at times more 

than a month old, a factor which became critical 

in November and December 1941, officials in OP20-G did have access to the same Japanese naval 

COMINT available to Kimmel at Pearl Harbor and 

Hart at Manila. 

On occasion, such as on 26 and 27 November (see Appendix C), COM-14 and COM-16 

COMINT summaries, because of their content, 

were sent to Washington as messages. These particular messages, though considerably less alarming 

than others issued by COM-14 during the October–November 1941 period, appeared at the same 

time as the famous “Winds” messages translations 

(see Appendix B) and contributed to the developing 

sense of crisis in Washington. Hawaii’s report for 26 

November 1941 was a comprehensive summary of 

the Japanese naval and air buildup assembling for a 

southern operation. It conveyed a distinct sense of 

alarm at events. Corregidor’s report for 27 November identified in even greater detail the existence of 

both a Japanese Southern Force and a Mandates 

Force, including several Japanese ground force units 

(Base Forces) in the Mandates. 

Hawaii’s picture of the Japanese buildup was 

not as complete as it might have been, based on the 

details developed in their earlier summaries. COM16’s message confirmed and enlarged on COM-
 43

Warning Messages 

were read and translated there, it is possible that in 

the Asiatic Fleet diplomatic messages were not considered by themselves to be a likely source of either 

strategic or tactical warning. 

Warning Messages 

Warnings based, at least in part, on the contents of Japanese diplomatic messages were 

in fact sent to the Hawaiian and Philippine commands on at least three occasions: 24, 27, and 29 

November 1941.134 It seems clear, however, that 

after July 1941, as a matter of policy and probably 

as a practical security precaution, no intelligence 

material directly from MAGIC was sent to Admiral 

Kimmel in Hawaii or to U.S. command officials in 

the Philippines.135

Receiving the actual Japanese diplomatic messages would have done neither Kimmel nor Hart 

any practical service, aside from their obvious value 

in pinpointing vital areas of Japanese policies and 

intentions. They contained no Japanese naval or 

military information. Messages between Tokyo and 

Washington largely concerned the ongoing negotiations between Secretary of State Cordell Hull; Japanese Ambassador Kichisaburo Nomura; Minister 

Reijiro Wakasugi; and, later, Saburo Kurusu, Japanese Ambassador Extraordinary. Messages between 

from this source.131 Little time and fewer resources 

were left over to attack JN-25, the code which, if 

read, would have provided operational details concerning the Japanese Strike Force. 

As soon as possible after the Purple machines 

became available to Army and Navy cryptanalysts, 

the English texts of all translated diplomatic messages were delivered to both the Office of Naval Intelligence and the Military Intelligence Service (MIS) 

each day. By agreement, OP-20-GZ was responsible 

for dissemination of these messages within the Navy 

(Secretary of the Navy Frank Knox; CNO Admiral 

Harold R. Stark; A/CNO Admiral Royal E. Ingersoll; ONI, Admiral Theodore S. Wilkinson; and 

Chief, War Plans Division) and to the White House 

for the president’s naval aide, Captain John R. Beardall.132 Alwin D. Kramer and Arthur H. McCollum 

of the ONI Far East Desk decided what translations U.S. policymakers would see each day.133 This 

arrangement was consistent with the dissemination 

rules laid down in the 1937 Orange War Plan. Similarly, MIS was responsible for dissemination within 

the War Department (Secretary of War Henry Stimson; Chief of Staff George C. Marshall; and Chief, 

War Plans Division) and to the State Department 

(Secretary of State Cordell Hull). 

In Hawaii, neither the Army nor the Navy commander had facilities for decoding Japanese diplomatic messages. Overall policy regarding dissemination of Japanese intercept by both G-2 and ONI 

dictated that MAGIC material based on diplomatic 

messages would not ordinarily be distributed to any 

commander outside Washington. The primary reasons for this policy were to protect the source and 

to retain in Washington the evaluation of purely 

diplomatic material. There was, however, no rule in 

either the War or Navy departments that prevented 

dissemination of MAGIC information to theater 

commanders. Facilities for decoding Japanese diplomatic messages, including messages in the Purple 

system (MAGIC), were available to Station C in the 

Philippines. However, if any diplomatic messages 

Overall policy ... dictated that 

MAGIC material based on 

diplomatic messages would not 

ordinarily be distributed to any 

commander outside Washington. 
44

Pearl Harbor Revisited

nese air and naval strikes against the Philippines, 

and this locale actually appeared in the warning 

messages of 24 and 27 November as one of several 

likely Japanese objectives.139

Reaction 

I

nexplicably, the warnings issued by Washington 

were virtually the only direct military actions taken 

that can be traced directly to COMINT despite the 

sense of urgency that COMINT reflected. Admiral Kimmel in Hawaii, though a recipient of ample 

warning on the approaching crisis, was not particularly alarmed by COM-14’s reports. Lacking any 

information on the Japanese 1st Air Fleet, and except 

for the warning messages, unaware of the content of 

messages in Japanese diplomatic channels, his attention was focused on the western Pacific. 

With regard to the Philippines, the sense of 

alarm, at least in U.S. Navy circles, was paramount. 

By mid-September 1941, Admiral Thomas C. Hart, 

Commander in Chief, Asiatic Fleet, since July 

1939,140 had become very concerned over the intelligence reports on Japanese naval activities supplied 

by COMINT and other sources. By November, 

Hart clearly saw, through his regular visits to Station C in the Malinta Tunnel on Corregidor, that 

his fleet would soon be at war and that “time was 

running out.”141 Surviving records, however, do not 

provide a clear connection between a COMINT 

cause and an operational effect in the Asiatic Fleet. 

Because of the general disagreement that prevailed among U.S. officials in Washington and the 

Pacific over U.S. objectives in the area, particularly 

with respect to British and Dutch possessions and 

the defense of the Philippines, Hart dispatched 

his fleet on a series of strategic deployments during September and October 1941 that first removed 

and then returned his forces to the Manila area. In 

September, convinced that the U.S. government 

would not defend the Philippines, Hart sent all his 

Tokyo, Washington, and other diplomatic posts 

frequently concerned Japanese espionage tasks and 

the efforts of diplomats to obtain information concerning U.S. naval and air dispositions in Panama, 

Hawaii, Manila, and various locations on the U.S. 

West Coast. Collectively, these messages conveyed 

an alarming interest in major fleet activity and an 

unmistakably hostile intention toward the United 

States. Their tenor deteriorated sharply after 26 

November when the United States delivered its tenpoint response to the Japanese note of 20 November. 

The messages did not, however, disclose the movements of the Japanese fleets.136 Only the unread, 

untranslated Japanese naval messages held this vital 

information.137

Despite the fact that all messages in Japanese 

diplomatic channels were not available by 7 December and that the daily reports mailed from Hawaii 

and Corregidor were at least two weeks en route, by 

late November 1941 U.S. Navy officials in Washington, Pearl Harbor, and Manila well knew that 

war with Japan was imminent.138 Made aware of 

hostile Japanese intentions by a profusion of intelligence (most of it COMINT), Admiral Stark, CNO, 

after 23 November 1941, repeatedly warned his 

Pacific commanders of impending Japanese attacks, 

placed restrictions on ship movements, and probably approved DNC’s orders to destroy codes. The 

weight of evidence overwhelmingly favored JapaCertainly COMINT can claim 

a major share of the credit for 

the fact that on 8 December 1941 

Asiatic Fleet losses were minimal.
 45

Reaction

wrote in his diary, “Guess war is just around the corner, but I think I’ll go to a movie.” The entry for 8 

December states, accordingly, “It [the attack] was no 

surprise by a matter of 18 hours.”144

Despite these rather tenuous indications of 

Hart’s responses, there is plenty of evidence from 

another source that Hart did in fact react aggressively to Japanese activities. Japanese radio intelligence 

messages from Taiwan between 17 November and 

3 December 1941, which were not read and translated until after the war in 1945–46, contained many 

reports of U.S. Army air and U.S. Navy air and ship 

reconnaissance. In addition, the Japanese consulate 

at Manila was very active in reporting the arrivals 

and departures of submarines and surface vessels.145

The Japanese perception of Army B-17 and 

fighter activity in the vicinity of Manila was one 

of declining activity. On 2 December 1941 Taiwan 

reported U.S. Army planes as “extremely inactive 

recently,” and on 3 December as “greatly reduced 

since 30 November ... Prior to 30 November, 10 or 

more planes per day heard; on the 30th, 1; on the 

31st, 2; none on the 3rd.”146

U.S. Navy reconnaissance of the airspace 

around Luzon increased during November–

December, according to Taiwan on 3 December 

1941. Taiwan reported the area patrolled by aircraft 

on 2 December as “300 miles south and southeast 

of Manila and west of northern Luzon.”147 In addition, the U.S. Navy was also active in surface patrols 

in the vicinity of Taiwan, according to the Japanese 

messages.148 In one of his many reports of visual 

observations, the Japanese consul in Manila, on 3 

December 1941, reported departures from Manila 

of possibly seven submarines and, from Cavite, 

the departure of the light cruiser Houston, all to 

unknown destinations. 

There is little doubt on balance that COMINT 

from Station C contributed to the Hart decisionmaking process. Certainly COMINT can claim 

a major share of the credit for the fact that on 

surface vessels to the south. On 27 October 1941, 

sensing a change in policy, Hart proposed to Washington that he join General Douglas MacArthur in 

defending the Philippines, and without first obtaining Washington’s approval, he brought the Asiatic 

fleet back to Manila. On 20 November 1941, Washington disapproved his plan. This forced Hart, at 

virtually the eleventh hour, to redeploy his surface 

vessels to southern Philippine and Netherlands East 

Indies ports. Patrol aircraft and submarines were 

retained in the Manila area. Under these circumstances, it is difficult to distinguish Hart’s deployments in response to intelligence from those taken 

in response to Washington. 

Lacking a declaration of war by the United 

States against Japan and keenly aware that the United States did not wish to appear to be the aggressor, Hart, since receiving the first war warning from 

OPNAV on 24 November 1941, had been “edging 

toward increasingly risky action.”142 Based on “intelligence intercepts,” Hart authorized air patrols over 

Japanese convoy movements along the China and 

French Indochina coasts. On 6 December 1941, after 

receiving confirmation that a Japanese amphibious 

force was steaming across the Gulf of Siam, Hart 

ordered one of his destroyer divisions to sail west 

from Balikpapan, Borneo, to Singapore. There the 

division commander was to place his ships under the 

British Fleet commander.143

Admiral Hart in his personal diary for the 

period 1–30 November indicated a preoccupation 

with two major problems: coordination of his own 

plans for defense of naval shore facilities with the 

U.S. air forces in the Philippines, and Japanese troop 

movements along the China and Indochina coast. 

He estimated that the information provided him 

represented primarily a threat to Thailand and to 

the British in Indochina. He believed he had taken 

all prudent measures in anticipation of an attack, 

although his diary referred to no specific actions taken in response to the warning messages he received 

on 24, 27, and 29 November. On 7 December he 
46

Pearl Harbor Revisited

both CINCAF and CINCPAC information on the 

whereabouts of the Japanese Fleet. 

At Pearl Harbor cryptanalytic emphasis shifted 

by mid-December from the Japanese Flag Officers Code and various shipping codes to recovering 

and exploiting the Japanese Fleet General-Purpose 

System. This was part of Safford’s regrouping of 

tasks and responsibilities between Washington and 

Hawaii after the events of 7 December revealed with 

painful clarity the type of information probably contained in the Japanese Navy’s messages and after the 

volume of diplomatic material declined. OP-20-G 

also recognized the tenuousness of its position in 

the Philippines and quickly put in motion plans to 

salvage Station C’s manpower. Some of the senior 

officers in the communication directorate took 

this opportunity to centralize control of the entire 

COMINT operation in Washington.150

8 December 1941 Asiatic Fleet losses were minimal: 

two amphibious patrol aircraft (PBY) and the gunboat Wake. Little if any of the COMINT provided by Station C came from cryptanalysis. Because 

Washington could not supply current code group 

meanings, Station C was not able to read messages 

in the Fleet General-Purpose System, JN-25, or in 

several of the minor naval codes.149

COMINT after the Opening Attack

Following the attack on Pearl Harbor, COMINT 

resources in Hawaii, after a two-day pause probably caused by the loss of contact with Heeia, resumed 

publishing a daily COMINT summary. This summary continued to follow the activities of the Japanese 

Fleet. Corregidor concentrated its efforts on supporting local Navy and Army commanders by providing 

warning of incoming air strikes as well as supplying 
47 47

Conclusion

cial Washington while it followed every fluctuation 

in the diplomatic negotiations between Ambassador 

Nomura and Secretary of State Hull. Judging from 

the conflicting guidance given to CINC Asiatic 

Fleet, abundant warning information produced by 

Hawaii and Corregidor from their analysis of the 

Japanese Navy’s communications system and the 

activity it reflected was apparently either ignored 

altogether or treated as unsubstantiated rumor lacking any supporting evidence from readable messages. It is certain, however, that COMINT based on 

Japanese Navy communications available to these 

Throughout its relatively short life, OP-20-G, both 

in Washington and in the Pacific, had suffered 

a lack of manpower. In the final months of 1941, the 

lack of overall manpower resources combined with the 

disposition of the available cryptanalysts resulted in 

the failure to read the critical messages of the Japanese 

Strike Force targeted for Pearl Harbor. Briefly recapped, 

two thirds (fifty-three) of the officer cryptanalysts were 

in Washington where, if they were assigned to technical positions, they were exploiting Japanese diplomatic 

messages, operating a twenty-four-hour watch and 

performing coderoom tasks which included running 

the Atlantic DF network, and conducting research on 

Japanese Navy cryptographic systems, e.g., JN-25. Less 

six officers in transit, the remainder were assigned in 

unequal proportions to Hawaii (twelve) and Corregidor (nine) where, in both stations, some were diverted 

to traffic analysis and machine room responsibilities. 

It may be argued that a more or less even distribution 

of collectors and DF operators between East Coast 

and Pacific stations was also a misalignment of critical 

resources, but it is clear that the placement and occupation of cryptanalytic personnel penalized Japanese 

Navy targets. 

It is clear too that, between September and midNovember 1941, the activities of the Japanese Navy, 

as it prepared for war, were also overlooked by offiThe decision to put the defeat of 

Germany in first priority led in 

June 1941 to cancellation of all 

war planning of any consequence 

in the western Pacific ...
48

Pearl Harbor Revisited

We will probably never know precisely why 

OP-20-G arranged its manpower resources in 

Washington and the Pacific as they were when 

the Japanese attacked Pearl Harbor and Manila. 

Undoubtedly, the new policies had left OP-20-G 

in an awkward position, creating new problems and 

aggravating old ones. Unwilling and perhaps unable 

to dismantle the COMINT edifice in the Pacific 

it had worked for twenty years to build, OP-20-G 

throughout 1941 let Hawaii and Corregidor perform the functions for which they had been prepared and trained while the workforce in Washington did its best to provide support in both theaters 

as well as to abide by its odd-even agreement with 

the Army concerning Japanese diplomatic messages. The historical manpower problems generated 

at least in part by a lack of respect for the intelligence profession were probably felt most keenly in 

Washington, where a decentralized management 

philosophy had been unable to prevent a concentration of more workload than the workforce could 

possibly achieve. The Washington center’s limited 

cryptanalytic resources were not, moreover, focused 

on the Japanese Navy where they belonged. As a 

result, they could not read any of the Japanese Navy 

cryptographic systems, and they became preoccupied with the Japanese diplomatic targets, which 

were providing unprecedented exposure for Army 

and Navy COMINT centers at the highest levels 

of government. Without minimizing the influence 

of strong interservice rivalry, the fact that OP-20-G 

was not concentrating its resources on Japanese 

Navy targets may suggest instead that a transition in 

emphasis toward the Atlantic by assigning more and 

more people to the twenty-four-hour watch had in 

fact already begun when the Japanese attack came. 

officials did not indicate that the Japanese intended 

to attack Pearl Harbor. 

In an attempt to place some restraints in the 

path of the Japanese government, U.S. military leaders, with the approval of President Roosevelt, agreed 

with the British to establish separate commands for 

the Philippines and the Southwest Pacific—the former under Admiral Thomas Hart, CINC Asiatic 

Fleet, the latter under the direction of the senior 

British officer. The principal U.S. goals in the western Pacific at this time were to avoid being drawn 

into the British plans to defend Singapore and to 

avoid antagonizing the Japanese government. The 

United States claimed that these arrangements, 

along with the presence of the U.S. Pacific Fleet 

at Pearl Harbor, were sufficient deterrence without 

representing a belligerent act. 

Viewed in retrospect, these circumstances very 

strongly suggest that both OP-20-G and the Chief of 

Naval Operations had been swept along by the same 

overpowering pull of events in Europe and the Atlantic and were confused by conflicting American and 

European objectives in the Far East. Because of the 

unexpected German successes in 1940 and early 1941, 

the entire U.S. military establishment was confronted 

by an abrupt shift in both political and military priorities which, in January 1941, had become partially 

institutionalized by the first American-British Conference (ABC–1). This conference established the 

primacy of the Atlantic theater over all war planning. 

The decision to put the defeat of Germany in first 

priority ultimately led in June 1941 to cancellation 

of all war planning of any consequence in the western Pacific and placed naval emphasis on the western 

Atlantic and in the Pacific east of 180 degrees. 
 49

Naval Messages Intercepted between 

5 September and 4 December 1941

Appendix A

This appendix contains seventy-two selected Japanese naval messages intercepted between September and 4 December 1941 by Navy intercept sites 

at Hawaii, Guam, and Corregidor; these messages 

were not decoded and translated until September 

1945–May 1946. At the end of the appendix are three 

diplomatic messages pertaining to the crisis that were 

translated on 8 and 30 December 1941. The Japanese 

naval messages are part of a total of 26,581 Japanese 

dispatches examined by U.S. Navy cryptanalysts.

The Japanese messages were originally discovered in a sanitized but unpublished group of 188 

messages contained in a document obtained from the 

Navy Archives at Crane, Indiana, by the then-NSA 

Historian Henry Schorreck. Subsequently, some of 

the messages were located among approximately 

2,400 translations given to the National Archives by 

NSA in 1978–79. Wherever this occurs, the appropriate reference number (SRN) is provided. Under 

these circumstances, it is not yet possible to verify 

in what Japanese cryptsystem each message was 

enciphered, nor have all the messages in the unpublished source been found in those released to the 

National Archives. (See particularly SRN-115202 to 

SRN-117840.)

It is interesting to note that the work of decrypting and translating these messages occurred at the 

same time that the congressional investigation of 

the attack on Pearl Harbor was being conducted. A 

review of the material provided witnesses before the 

congressional committee and to the committee itself 

by the Navy’s Pearl Harbor Liaison Office, however, 

indicates that these messages were not made available to the Liaison Office. Ship identifications were 

taken from Conway’s All the World’s Fighting Ships, 

1922–46 (New York: Mayflower Books, 1980) and 

organizational relationships from The Imperial Japanese Navy in WWII, prepared in 1952 by U.S. Army, 

Far East Command, Military History Section.

The reader will note a 27 November 1941 message from Imperial Headquarters to the Striking 

Force containing the names of two Russian freighters, the Uzbekistan and the Azerbaidjan. The messages announced their presence in the Northern Pacific. 

Unhindered by restrictions of any kind, they were 

probably communicating with their owners in the 

Soviet Union. Their signals would have neatly supplied the basis for Seaman Z’s sensational revelations 

that he had tracked the Japanese Striking Force’s 

radio signals as it steamed toward Pearl Harbor. This 

episode is outlined in many secondary sources but 

most recently by John Toland and Edwin Layton in 

their books, Infamy: Pearl Harbor and its Aftermath

and And I Was There, respectively.
Pearl Harbor Revisited

50

6 Oct

From: 1stAirFltStf

Action: CdrCarDiv2

CdrKagoshima Air Grp

In 1st Air Flt aerial torpedo attack 

drill #13, which was to be conducted on 21 October against BatDiv1, 

Akagi and Kaga were each allotted 

nine torpedoes and Soryu and Hiryu

are each allotted six torpedoes. 

SRN-117453

13 Oct

From: staff combined

To: all flags

This message concerned the first 

combined fleet communications drill. 

It contained the first references 

to a striking force and advanced 

expeditionary force and mentioned 

dispatches suitable for x + 17, x + 

30, and x + 45 days.

21 Oct

From: Tokyo Comms

To: ALLNAV

This message revised a Navy call 

list by adding callsigns for new 

warships over a year before commissioning the Yamato (bb) and the 

Agano (cl).

5 Sep

From: COS2

To: 2nd Fleet

Info: COSCombined

“a state of complete readiness for

battle operations must be achieved by

the first part of november ... exert

even greater efforts toward achieving

maximum fighting strength ...”

9 Sep

From: COSCombined

To: All Fleet COS; All Flt CINCs

“as conditions become more and more

critical, each and every ship and

unit will aim at being fully prepared

for commencing war operations by the

first parts of november ... completing

all personnel changes ordered 3 august

asap.” SRN-115533

4 Oct

Between 4 October and 15 November the 1stAirFlt conducted almost 

daily drills at fueling at sea. 

The messages were most revealing 

in that their addressees actually 

showed the force structure of the 

Strike Force and their respective 

Trains (Tankers).
 51

appendix A

27 Oct

From: Staff1stAirFlt

To: Staff BatDiv3

Message arranged torpedo launching 

exercise for CarDivs1/2/5.

28 Oct

From: COS1stAirFlt

[Three messages to and from 1stAirFlt reinforced idea that CINC1stAirFlt was commander of Strike Force.] 

“on 30 october this fleet will

pick up from 5-10 (near surface) 

torpedoes at sasebo ... classes on this

torpedo will be held at kanoya for

about five days from the 31st and

then (emphasis) will be shifted to

firing practice. by working night and

day it should be possible to complete

100 (attachments for torpedoes, 

probably bow or stern planes) by

5 november.” srn-117301

28 Oct

From: COS1stAirFlt

“as the increase in the number of torpedoes handled and the lack of personnel

is causing grave delays in torpedo adjusting, please make special arrangements

to send to this fleet from yokosuka air

group—about 60 torpedo adjustment personnel to assist in the work of adjusting torpedoes between the end of october and 20 november—40 to kanoya for

cardivs1/2—20 to ooita air for cardiv5 

... please arrange increase of one torpedo officer to each of the carriers in

cardivs1/2/5.” srn-116323

22 Oct

From: COS Sasebo SND

“warship agano launched ... today.” 

SRN-116139

24 Oct

From: 1stAirFlt staff

To: Staff6flt, Staff Desron1, Staff

Batdiv3, Staff Crudiv8, 1stAirFlt, 

Cardiv4, Tokyo comunit, Tokyo DF 

control, Staffcombined

This message announced the second 

combined fleet comm test and again 

cited the “Striking Force.” Possible leading role for 1stAirFlt in 

Striking Force. SRN-117089

26 Oct

From: COS1stAirFlt

To: CruDiv8, DesRon1, SubRon1

This message requested that all 

ships scheduled to be assigned to 

the (Striking Force?) have all torpedoes adjusted by 18 November. 

SRN-116684

27 Oct

From: COS1stAirFlt

Experience has shown that in operations the Train frequently preceded 

the Main Body. Accordingly, Trainrelated messages were important. 

This message identified four tankers 

to be assigned 1stAirFlt and to rendezvous at Sasebo by 10 November.
Pearl Harbor Revisited

52

1 Nov

From: CINC1stAirFlt

“after completing battle preparations

tooei maru will obtain about 750 

drums of fuel oil (for use of akagi) 

and 12,000 kerosene tins of fuel

oil (for hiryu) from yokosuka ... and

rendezvous at sasebo 10th ...” 

SRN-117150

1 Nov

From: CdrSubRon6

To: CINC2

“... kinu and yura will ambush and

completely destroy the u.s. enemy.” 

SRN-117001

2 Nov

From: Navy Minister

To: Yokosuka ND

“have air depots 2 and 11 supply

live bombs to akagi, soryu, hiryu, 

shokaku, and zuikaku to ascertain

their capabilities ...” SRN-117665

3 Nov

A message from the Chief of Naval 

Technical Bureau, General Affairs 

Section, to the Yokosuka, Sasebo, 

Kure, and Maizuru Yards emphasized 

that work on transport vessels be 

completed by 20 November.

28 Oct

From: Yokosuka ND

“send personnel and workers to carry

out instructions on type 91 torpedoes

(equipped with stabilizers) ...” 

SRN-116476

28 Oct

COS1stAirFlt

On 27 and 28 October arranged for 

two tankers to join his fleet by 

about 10 November—the Shinkoku Maru 

and Toohoo Maru. In the meantime 

these tankers were being equipped 

for refueling under tow.

30 Oct

From: COS1stAirFlt

Action: Kuroshia

Maru or Shinkoku Maru

Info: Kure NavyYrd

Kure Sanda Section

CdrCarDiv2, U/I

“when installation of gear for

refueling undertow and preparations for

action have been completed, kuroshio

(koku/choo) maru and shinkoku (kami/

kuni) maru will depart sasebo and

kure, respectively, on the 13th and

proceed to kagoshima bay, conducting

exercises with carriers en route. 

request they load fuel oil for

refueling purposes before they depart.” 

SRN-116588
 53

appendix A

5 Nov

In addition to refueling from tankers 

the Strike Force would be required 

to carry a deck cargo of oil drums. 

On carriers, the extra weight of the 

drums was cause for concern as shown 

by the following advice to COS 1st 

Air Fleet from the Chief, Bureau 

Military Affairs Section: 

“regarding the loading of drums of

fuel oil on ships of your fleet ... it

will affect the strength of the hull

and the ship’s performance.

a. amount to be loaded: akagi, under

600 tons; boryu and hiryu under 400 

tons, and an equivalent weight will be

removed.

b. in the case of akagi and hiryu, 

load amidships and avoid bow and stern.

c. in the case of boryu, load evenly

over length of ship.

d. we have prepared 1400 tons...” 

SRN-116566

9 Nov

From: CINC1stAirFlt

To: CdrDesRon

The fact that major combat elements 

of the Strike Force were to be at 

sea on 13–14 November was revealed 

in two messages directing fueling 

at sea exercises for four Maru vessels with Akagi, DesRon1, CruDiv8, 

CarDiv2 and CarDiv5 and from 

CdrDesRon1 directing that the Hiryu

and a tanker drill on the 15th off 

Ariake Bay (extreme southern Japan). 

SRN-115709, 115784

3 Nov

From: Staff1stAirFlt

To: CdrSaekiAirBase

“in the 3rd special drill in

ambushing, 54 shipboard bombers will

carry out a bombing and strafing

attack in sight of saeki base from

0815 on the 4th, 0715 on the 5th, 

and 0815 on the 6th, and about an

hour or hour and a half afterwards 54 

shipboard attack planes will carry out

a similar bombing attack.” 

SRN-117665

4 Nov

From: CdrCarDiv2

“cdrdesdiv23 will dispatch yuuzuki

(dd) to saeki to arrive about 0700 

the 6th. yuuzuki will pick up and

take to kagoshima (4) torpedoes

which cardivs1/2 are to fire against

anchored capital ships on the morning

in question ...”

5 Nov

The equipment for refueling under 

tow at sea included special fenders 

0.9 mm in diameter, and the requisite lines for attaching them to 

the tanker. On this date a message 

listed 10 Maru vessels, at least 

five of which were firmly associated 

with the Striking Force. SRN-117031 
Pearl Harbor Revisited

54

12 Nov

From: CdrDesRon1

This message suggested that the 

entire Strike Force did not depart 

Inland Sea on 10 November: 

“in view of the scheduled operation of

this unit it is desirable to complete

loading of antiaircraft ammunition and

fuses ... for abukuma, (cl, desron1 

flagship) desdiv17, desdiv15, and

akigumo (dd launched 4 november 1941 

associated with cardiv 5 by (13 

november) ...” SRN-115543

12 Nov

From: TokyoNGS Scty

To: SctyCombined

Info: Scty1stAirFlt

The following message seemed to 

firmly identify CINC1stAirFlt as 

Strike Force Cmdr:

“please supply the striking force with

copies of the special area designator

list (issued by ngs) from those which

have already been (secretly) issued to

your headquarters.” SRN-115381

14 Nov

From: Navy Minister

To: ALLNAV

“the publication wartime recognition

signals for japanese merchant shipping

will be placed in effect 1 december

1941 ...” SRN-115380

10 Nov

From: CINC6

To: 6thFltSubs

Info: AllFltCINCS

“maintain wartime radio silence on

shortwave commencing 0000 november

11.” SRN-117687

10 Nov

From: COSKureND

Info: COS1stAirFlt

“arrangements have been made to

(reequip) four maru vessels for

simultaneous port and starboard

refueling by 13 november.” 

SRN-117258

11 Nov

From: CINC1stAirFlt

This message assigned anchorages at 

Saeki Wan (western Inland Sea) to 

the capital ships and tankers of 

the Strike Force. SRN-115787

12 Nov

From: COS Combined Flt

Action: NavyTechBur Kure Navy Yrd

“in view of the necessity for

completing by 17 november the

installation for refueling at sea...” 

SRN-116589
 55

appendix A

16 Nov

From: SctyCombined

To: Flagships

Two messages revealed details of 

designator list and scope of forthcoming fleet operations:

 “revision #1 to navy call list

#9 effective november 15: striking

force, submarine force (southern

force), maru force, communications

force, commerce destruction force, 

advance expeditionary force, supply

unit ... (for each force), southern

force, northern force, south seas

force, e force (british malaya)*, 

h force (dutch east indies)*, m

force (philippines)*, attached

force, g occupation force (guam)*, 

aa occupation force (wake)* and af

destruction units (midway).*”

(*ca 1942) SRN-115430, 116430

[The next seven messages were pivotal in locating the Strike Force.]

17 Nov

From: Chief 1st Section NGS

To: COS1stAirFlt

Info: CdrBatDiv3

(BB HIEI was 1st section of 

BatDiv3.) 

“suzuki (1776) is being sent to your

hq on board hiei to report inspection

results.” SRN-116436

14 Nov

From: CdrCarDiv5

To: CINC1stAirFlt

“flagship was changed to zuikaku at

0830 on 14th ... 1st section, zuikaku

and akigumo; 2nd section shokaku and

oboro.” SRN-115712

14 Nov

From: CINCCombined

To: 7 Marus

Message assigned 5 Maru vessels to 

CINCS1stAir, 2ndFlt, and 4thFlt. 

SRN-115785

15 Nov

From: Tokyo Bureau of Mil Prep

Message assigned Akebono Maru to 

1st Air Flt.

16 Nov

From: CINC1stAirFlt

To: CdrDesDiv15, CdrDesRon1, 

CdrSubRon1, 1stAirFlt (less

CarDiv4, CarDiv3), CdrBatDiv3

Info: CINCSAllFlts

Tokyo DF Control

Hdg resolved question of who commands and composition of the Striking Force: 

“strike force opord#1: commencing

0000 19 november, ‘battle control’ 

effective for short wave frequencies

and ‘alert control’ for long wave.” 

SRN-115397
Pearl Harbor Revisited

56

19 Nov

From: ProbSubUnit

To: CINCCombined, CinC1stAirFlt, 

CdrTokyoComUnit, CdrYokosukaComUnit, 

CINC6, CdrOminatoComUnit

“... until 2000, the 20th, yokosuka

comm zone. until 0800, the 22nd, 

ominato comm zone. thereafter, 

1stairfltflagship comm zone.” 

SRN 117673

19 Nov

From: ProbSubUnit

To: CINC1stAirFlt, CINC6, 

CdrTokyoComUnit, CdrOminatoComUnit, 

CdrYokosukaComUnit

“... until 2000 the 20th yokosuka

comm zone. from 2000 the 20th

until 0800 the 22nd, ominato comm

zone. thereafter, 1stairflt flagship

comm zone.” (Ominato is approx 

41N–141E). SRN 117666/117674

20 Nov

From: StaffSubDiv2

To: CdrOminatoComUnit, 

YokosukaComUnit, Staff6FLT, 

Staff1stAirFlt

“i-19 will leave yokosuka comm zone

on november 21, and enter ominato

comm zone. at 1600 november 22 will

enter 1stairfleetflagship comm zone.” 

SRN 116329/116990 

18 Nov

From: Chief 1st Section, NGS

To: COS Ominato Guard District

Info: COS1stAirFlt

“please arrange to have suzuki

(1776) who was sent to the 1stairflt

on business, picked up at about 23 or

24 november at hittokapu wan by ___ 

of your command.”

(Hittokapu Wan is located in the 

Kurile Islands at about 45N-147-40E). 

SRN 116643. See also Prange, At Dawn 

We Slept, Ch. 43, 342–352, for identity of Suzuki and his mission. 

19 Nov

From: COS Ominato Guard District

To: NGS, 1stSecCh

“he (suzuki) will be taken aboard the

kunajiri (ca).” SRN 116920

19 Nov

From: CINC Comb Flt

To: All Flagships Combined Fleet

“the fourth series of communications

tests for the combined fleet will be

held as follows: (on november 22 

and 23). 2. participating forces: 

combined fleet (however, the forces

presently en route to the standby

location will not receive test). 3. 

principal topic for consideration in

this drill: investigation and study of

the communications setup required to

effectively handle the situation upon

opening up of hostilities...”SRN 115678 

and Prange, At Dawn We Slept, 332.
 57

appendix A

20 Nov

From: Shiriya

Action: COSCarDivs, Combined Flt

“1. will complete loading fuel

oil (aviation gasoline) and other

miscellaneous equipment 21 november. 

2. the main generator and other minor

repairs will be completed 23 november. 

3. expect to get underway on the

24th and join up during morning of

27 november. 4. striking force secret

operation order #6 has not been

received.” SRN-115375

21 Nov

From: Navy Minister

To: All Majcoms Afloat

[Radio calls are normally effective for about six months. The last 

change was on 1 November. The following message is noteworthy.]

“commence using call list #10 from 1 

december and discontinue using list #9 

as of 30 november.”

21 Nov

From: C.O. Shiriya

To: CdrDesDiv7

“we are now undergoing overhaul at

yokosuka. expect to depart november 24 

and arrive sea on november 27. advise

rendezvous point with your unit. 

regard fueling at sea ...”

(DesDiv7 is escort of CarDiv1, 

Akagi and Kaga)

20 Nov

From: CINCCombined

To: CINC2nd, CINC3rd, CINC4th, 

CINC11thAirFlt, CENC1stAirFlt, 

SaseboComUnit, Southern Expeditionary

Fleet (Less SubRon/6)

“... at 0000 on 21 november repeat

21 november, carry out second

phase of preparations for opening

hostilities.” SRN-115385

20 Nov

From: CINC11thAirFlt

To: 11thAirFlt

“commencing 20 november, when planes

(or plane units) are shifted, maintain

precautionary shortwave silence.”

20 Nov

From: Hainan Guard District

To: Hainan Force Staff

“report of leakage of secret

information regarding the concentration

of our troops on hainan island to a

foreigner. commencing 24 november no

one will be permitted to leave or

enter hainan island ...” SRN-115438
Pearl Harbor Revisited

58

25 Nov

From: CINCCombined

To: All Flagships

“from 26 november, ships of combined

fleet will observe radio communications

procedure as follows:

1. except in extreme emergency the

main force and its attached force will

cease communicating ...” SRN-116866

25 Nov

From: Imperial Hq, 1stSecNavSecCh

To: COS2, COS3, COS4, COS11th

Air, COSSEF, COSChina Area Fleet, 

All ND COS

“(plans) for exhaustive conscription

of ... and civilians are in hands of

central authorities. in order to

preserve security, however, they will

be activated at a future time ...” 

SRN-116908

26 Nov

From: COS2

Anticipating casualties the Navy 

arranged for hospital facilities 

for the end of December.

“complete all necessary arrangements

for the hospitalization of 1,000 

patients each at bako (including

takao), sama, and palau ... be prepared

to supply ten times the annual

‘battleship requirements’ of medical

supplies for dressing of wounds and

disinfection by 10 february 1942.” 

SRN-115439

22 Nov

From: PossCarDiv4

Kasuga Maru and Hokaze (DD) CarDiv4 

ordered to transport planes from 

Sasebo to Palau ASAP.

25 Nov

From: Chief, 1st Section, Naval

Sec, Imperial Hq

The Japanese had maintained a close 

interest in the U.S. Marines at 

Shanghai and Tientsen for several 

weeks and knew they would be evacuated on the American liners President Madison and President Harrison. This message ordered COS China 

Area Fleet to report time of departure by urgent dispatch to 2nd and 

3rd Fleets, 2nd China Expeditionary 

Fleet, and Bako Guard District. See 

SRN 116737, 27 November.

25 Nov

From: COS2

To: COSCombined

Info: COS3

“... since we have assigned all eight

patrol boats to the philippine force

and orders for their use have already

been issued ...” SRN-116910
 59

appendix A

27 Nov

From: Navy Minister

To: All Majcoms, CINCall NDs, 

CINC All Guard Districts

“from now on all merchant shipping, 

all naval comm units, and naval

shipping will stand radio guard

(listening watch) as set forth

in articles 12 and 13 of secret

communication regulations for merchant

shipping.” SRN-115636

28 Nov

From: Imperial Hq., NGS

To: All Majcoms

“beginning 1 december 1941, tokyo

comm unit will initiate broadcasts on

... 4175kc in order to (maintain) 

volume of traffic ... afloat, etc., in

accordance with principles given in 2nd

communications analysis of 1941.” 

28 Nov

From: Ch1stSec,Nav Sec, Imperial Hq.

To: All Majcoms, All NDs, All

Guard Dist, Tokyo Hydro office

“commencing this date, in special

weather reports sent from (this

office) locations will be indicated by

navy grid chart–afloat weather list.” 

SRN-115456

26 Nov

From: ComCarDiv3

To: CINC2

Info: CINC3

“in view of this force’s operations

and future we definitely desire to be

refueled before arriving at palau ...”

27 Nov

From: 1stSecNavSec, Imperial Hq.

To: Striking Force, COSCombined

“although there are indications of

several ships operating in the

aleutians area, the ships in the

northern pacific appear chiefly to be

russian ships ... they are uzbekistan

(about 3,000 tons ... 12 knots) and

azerbaidjan (6,114 tons less than

10 knots). both are westbound (from

san francisco).” SRN-116667

27 Nov

From: TokyoComUnit

To: All Fleets

From: 1stSecNavSec, Ch,Imperial Hq

To: Striking Force, COSCombined

“1. weather report. the low pressure

center of 740 mm which was near ‘n

ri 0 na’ today at noon, is advancing

at a speed of 45km. wind speed is

over 15m, within 1000 km to sw of

center and about 26 m near the

center. the high pressure center in

‘ru’ area continues to proceed eastward at a speed of 45 km. 2. prince

hiroyasu fushimi sends the following

msg to cinc nagumo: ‘i pray for

your long and lasting battle

fortunes.’” SRN-116668
Pearl Harbor Revisited

60

30 Nov

From: COSBakoGuardDist

Action: COSCombFlt, COS2ndFlt, 

COSSouthChinaFlt

“at 0200 the 30th, auxiliary gunboat

aso maru observed three american

minesweepers (650 tons) of __ class

at point 10 miles on a northeasterly

course. in reference to __ #81, it

is believed that the pescadores and

takao areas are being reconnoitered. 

we are attempting to confirm these

movements.” SRN-117290

30 Nov

From: ChTokyoComOf

To: Striking Force

Info: COSCombined

“at noon on the 30th, a high pressure

area of __ mm located in “yu,” “re,” 

“tsu,” “ho,” “u” blocks with other

areas quiet on the whole ... conditions

will continue for about two more days. 

at 0900 on the 30th there was a five

meter west wind in ai (oahu*) and rain

at af (midway*).” (*ca 1942) 

SRN-115460

1 Dec

From: COSSaseboND

To: CdrOkinawa, Area Base Force

“we have received word from naha

customs that the philippine registered

ship kurobeerugoo (kana) (44-ton) 

arrived in naha on 30 nov. seal her

radio at once—delay departure of this

ship—prevent their learning of our

activities.” SRN-117693

28 Nov

From: Ch1stSec,Nav Sec,Imperial Hq

To: Striking Force

“at noon on the 28th a high pressure

area of __ mm located in ‘tsu’ and

‘he’ sectors moving ese ... 55 kmh. 

another high pressure center of about

the same pressure in ‘u’ sector is

almost stationary ...” SRN-115690

29 Nov

From: CINC4

“all capital ships, destroyers, 

submarines of the south sea force and

the kujokawa maru are to maintain

battle condition short wave silence, 

starting 1200 nov 29.” SRN-115435

29 Nov

From: (6thBasFor)

To: CdrGdDiv#52, CdrGdDiv53

Info: DesRon6, Cdr4th lt

“the following forces are to be added

to the special landing forces for the

‘u’ occupation operations ...” 

SRN-115396
 61

appendix A

2 Dec

From: NavMinister

To: All ND CINCS, All GD

CINCS, All Fleet CINCS

“starting 4 december 1941, system

#8 of naval code will be used and

system #7 discontinued. (list 7 

will still be used with some japanese

stations.)” SRN-116741

2 Dec

From: Tokyo (Togo)

To: Honolulu

In connection with the CarDiv2 message of 4 November 1941 regarding preparations to fire torpedoes 

against anchored capital ships, the 

following message in a naval attaché system was particularly noteworthy. This message was received 

in Washington on 23 December 1941 

and translated by the Army on 

30 December 1941. (Connorton, 

Appendix 1, 194, item 87) 

“in view of the present situation

the presence in port of warships, 

airplane carriers, and cruisers is

of the utmost importance. hereafter, 

advise me whether or not the warships

are provided with anti-[torpedo] 

nets.”

3 Dec

From: CINCCombined

To: Combined Flt

“from 0000, dec 4th change ship

frequency system to #1 ...”

1 Dec

From: Shiriya

To: ComDesDiv7

“this ship is proceeding direct to

position 30-00n, 154-20e. expect

to arrive that point at 1800 on

3 dec. thereafter will proceed

eastward along the 30 degree north

latitude line at speed of 7 knots.” 

SRN-115398

2 Dec

From: CINCCombined

To: Combined Flt

“this dispatch is top secret. this

order is effective at 1730 on

2 december. climb niitakayama

1208, repeat 1208.” (climb mount

niitaka december 8) 

SRN-115376

In late 1945, possibly with knowledge in hand that this message was 

stipulated in Flt OPORDER#1, its 

meaning is understood by OP-20-G to 

be, “Attack on 8 December.” In the 

congressional investigation this 

message was incorrectly reported as 

sent on 6 December 1941. (Hearings 

Part 1, 185)
Pearl Harbor Revisited

62

6 Dec

From: Honolulu

To: Tokyo

[Regarding the torpedo net message 

of 2 December in diplomatic channels the following was translated 

by the Army on 8 December 1941]: 

“...in my opinion the battleships do

not have torpedo nets ...”

5 Dec

From: BurMilPrep, Tokyo

To: NavAttaché, Washington, Mexico

[Only one message was found after 

0000 4 December 1941 in the old 

cipher which could have been read 

before 8 December.] 

“copy being sent by wire to naval

attaché london. dispose of the cipher

machine and all of its rules for use

at once.”
63 63

Appendix B

Summary of Diplomatic Messages, July–November 1941

After 24 November 1941, events in U.S.-

Japanese diplomatic negotiations moved very 

swiftly to their climax on 7 December. A number 

of important diplomatic messages passed between 

Tokyo and Washington between July and November; these are summarized below. These early messages and those exchanged after 24 November that 

have been selected for inclusion in this appendix are 

so revealing that it is easy to lose sight of the fact 

that U.S. officials were often reading these messages 

at about the same time as the Japanese diplomats. 

The “War Warning” messages sent by OPNAV 

beginning on 24 November have also been included in this appendix to ensure that the reader fully 

appreciates their correlation with events occurring 

in diplomatic circles.

•	 Despite changes in its government, Japan 

remained committed to the Tripartite Pact 

with Germany and Italy.

•	 Japan frequently expressed determination to 

use force against the United States and Great 

Britain.

•	 Japan established an espionage network in the 

United States.

•	 Plans for evacuation of Japanese diplomatic, espionage, and newspaper personnel were discussed.

•	 Germany and Italy applied pressure on Japan 

to provoke war with the United States.

•	 Japanese attitude toward the U.S. Open Door 

policy hardened after 16 October when Tojo 

took over the government. Japan wanted the 

United States to approve Japanese policies in 

the Far East—including China and French 

Indochina—and restore Japanese trade status 

with the United States.

•	 Ambassador Nomura’s attempt to resign on 

22 October was refused.

•	 On 4 November, Ambassador Saburo Kurusu, 

sent to help Nomura, was not optimistic that 

negotiations would be successful. He arrived 

in Washington on 17 November.

•	 On 5 November Tokyo established a 

25 November deadline for completion of 

negotiations.

•	 Nomura reported on 10 November on statements from high-ranking politicians and cabinet members (a) that the U.S. was not bluffing, 

(b) that it was ready for war, (c) that it had reliable reports that Japan would be on the move 

soon, and (d) that the president and secretary 

of state believed these reports.

•	 On 19 November 1941, two messages from 

15 November were read which discussed plans 

to evacuate Japanese citizens from the United 

States. 
Pearl Harbor Revisited

64

The messages that follow are arranged in order 

of transmission. 

Army messages are indicated with an “A” and 

Navy messages with an “N.” The date given is the 

date the message was translated. 

N 25 Nov A circular message from Tokyo to 

Washington on 15 November with 

detailed instructions on how to 

destroy code machines.

N 28 Nov A circular message from Tokyo to 

Washington on 19 November with 

detailed instructions to listen for 

“Winds Execute” messages to be 

added to Japanese news broadcasts 

in case of diplomatic emergencies 

involving the United States, England, 

or Russia. When heard, embassies 

were to destroy all codes, papers, etc.

N 26 Nov A circular message from Tokyo to 

Washington on 19 November, sent 

after above message but translated 

earlier, contained instructions to listen for an abbreviated “Winds” message in general intelligence broadcasts 

repeated five times at beginning and 

end, i.e., only the word East, West, or 

North would be spoken five times.

A 28 Nov Circular message from Tokyo on 

20 November said U.S.-Japanese situation would not “permit any further 

conciliation by us” and rejected all 

feelings of optimism.

A 22 Nov Tokyo informed Washington on 22 

November that, by 29 November if 

agreement had not been reached, 

“things are automatically going to 

happen.”

 24 Nov OPNAV message warned of possible 

Japanese “aggressive movement” toward 

Philippines, Guam, or any direction.

A 26 Nov Tokyo message to Washington on 26 

November contained telephone brevity code to be used because “telegrams 

take too long.” The code covered topics under negotiation, situations, and 

personalities.

27 Nov OPNAV WAR WARNING message.

A 29 Nov Message on 26 November from 

Nomura to Tokyo recommended that 

Japan break diplomatic relations with 

the United States in a formal manner 

rather than “enter on scheduled operations” without prior announcement particularly since “our intention is a strict 

military secret.” A formal break would 

avoid responsibility for the “rupture.”

N 2 Dec A circular message from Tokyo on 

27 November contained another 

brevity code in which codewords 

were assigned specific meanings, e.g., 

“Japan’s and USA’s military forces 

have clashed” equals, “HIZIKATA 

MINAMI.” 

N 28 Nov A telephone conversation on 27 November between Washington (Kurusu) and 

a foreign office official in Tokyo named 

Yamamoto. Tokyo used telephone code 

to convey a message referring to an 

attack on the United States.

29 Nov OPNAV WAR WARNING message. 

Text indicated Army had also been 

notified. 
 65

A 1 Dec Message from Tokyo to Berlin on 

30 November directed the Japanese 

ambassador to inform Germany that 

U.S. relations had ruptured and that 

“war may break out quicker than 

anyone dreams.” Regarding Russia, 

Tokyo stated that if Russia reacted to 

her move southward and joined hands 

with England and the United States, 

Japan was “ready to turn on her with 

all our might.” Tokyo requested the 

Germans and Italians to maintain 

“absolute secrecy.”

N 1 Dec Message from Tokyo to Washington 

discussed means of allaying U.S. suspicions regarding Japanese reactions 

to the U.S. proposal of 26 November. 

News media were to be advised that 

“negotiations are continuing.” A plan 

was discussed to make a formal presentation in Washington vice Tokyo. 

The message queried president’s 

reaction to Tojo’s bellicose speech.

N 1 Dec A circular message from Tokyo on 

1 December advised Washington that 

London, Hong Kong, Singapore, and 

Manila had been instructed to destroy 

code machines.

 

2Dec OPNAV instructed CINCAF to establish defensive patrols. 

A 4 Dec Message from Rome to Washington on 2 December said that Tokyo 

believed the Hull note of 26 November “absolutely unacceptable,” and “a 

conflict(?) in the near future is considered very probable.” Rome also 

said Tokyo believed American Navy 

in Pacific was “not strong enough for 

decisive action.”

N 3 Dec Message from Tokyo to Washington 

on 2 December instructed Washington to burn all codes except one copy 

of the codes being used in conjunction with the machine (i.e., PURPLE), the O Code, and the abbreviation code. Washington was also to 

burn messages, other secret papers, 

and telegraphic codes, and possibly to 

destroy one machine.

 3 Dec An OPNAV message regarding Japanese instructions to burn codes.

N 6 Dec Messages from Berlin and Rome to 

Tokyo on 3 December described Japanese attempts to obtain German and 

Italian assurances that they would 

follow the Japanese declaration of 

war on the United States with their 

own. Hitler was not available, but 

Mussolini agreed. 

4 Dec OPNAV ordered U.S. codes destroyed.

N 6 Dec Washington confirmed destruction of 

codes on 5 December.

N 6 Dec Tokyo message on 5 December ordered 

four individuals in Washington to leave 

immediately. The translation contained 

a note which identified one as head 

of Japanese espionage in the Western Hemisphere and the others as his 

assistants.

A 6 Dec Tokyo message to Washington on 6 

December alerted Nomura that a formal reply to the 26 November note 

had been prepared, was very long, and 

would be in 14 parts. 

Messages in this appendix are from Radio Intelligence Publication No. 87Z, “The Role of Radio 

Intelligence in the American-Japanese Naval War,” 

Vol. I, Section A, by Ensign John V. Connorton, 

USNR. SRH-012, RG 457.

Appendix B
66

Highlights from COM-14 Daily COMINT Summary

Appendix C

This appendix represents abstracts taken from 

the daily COMINT summaries published by 

COM-14. To aid the reader in correlating these 

highlights with actual daily events, I have included salient extracts from Morison’s History of U.S. 

Naval Operations. To show the correlation between 

COMINT and warnings issued by Washington, I 

have also inserted, at the appropriate times, diplomatic and OPNAV messages.

The sources are indicated as follows:

“S” indicates COM-14 Traffic Intelligence Summaries, July–December 1941, SRMN-012.

“C” indicates Radio Intelligence Publication 

Number 87Z, The Role of Radio Intelligence in the 

American-Japanese Naval War, Volume I, by Ensign 

John V. Connorton, USNR (SRH-012).

“M” indicates Volume III, The Rising Sun in the 

Pacific, 1931–April 1942, by Samuel Eliot Morison.

Edited copies of the COM-14 Daily Summaries 

for the period 1 November–6 December 1941 may 

also be found in PHA, Part 17, 2601–42.

S 16 Jul Combined Air Force concentration 

in Takao [Taiwan] included in 3rdFlt 

addresses. Indications it will move 

south, i.e., to Taiwan from Japan. 4thFlt 

concentrating in Mandates [Defense?] 

Hiryu and probably other carriers concentrating in Taiwan area “awaiting the 

assumed Southern Operations.”

S 31 Jul New Task Force Formed—CINC3, 

China Flt, South China Flt.

S 20 Aug AirRon4 move indicated by heavy 

traffic.

S 21 Aug Commander AirRon24 at Saipan.

S 24 Aug New carrier, Shokaku—appeared in 

traffic. [Confirmed by COM-16 on 

28th.]

M Sep Training by carriers and air groups 

for Pearl Harbor attack began in 

September.

S 8 Sep New carriers Shokaku and Zuikaku being 

fitted out. [The Shokaku completed fitting out on 8 August 1941, the Zuikaku

on 25 September 1941.] Plane complements for all carriers are being completed. 

S 12 Sep COM-16 [confirmed] a new force 

being organized, possibly a 5thFlt.

S 18 Sep Heavy air-related traffic suggests air 

movement into Mandates forthcoming.
 67

Appendix C

S 14 Oct Movements to Mandates noted by 

Yokohama, Chitose, and Yokosuka Air 

units.

S 15 Oct 5thFlt is formed. 

S 16 Oct Communications network is expanding particularly in air-related communications. Callsign usage same as 

for “Temporary Shore” or “Advanced 

Base” stations associated with South 

China, Indochina campaigns.

S 16 Oct CINC Combined Fleet may have 

returned to Nagato. Associated with 

1st/2ndFlts, carriers, and subs. Chitose Air en route Mandates, Yokohama Air at Truk, Yokosuka Air possibly Palau area. Evidence that Navy is 

taking over Maru vessels and issuing 

callbooks and organizing into units.

S 17 Oct Tokyo relayed message from Spratly 

Island [South China Sea] to Takao 

[Taiwan] under priority procedure to 

COS Combined Flt, ComCarDiv4, 

CarDiv4, Intel unit Bako (Taiwan), 

COS South China Flt, COS French 

Indochina Force (Southern Exped. 

Force), Resident Naval Officer (RNO) 

Taihoku [Taipei], Cdr 11th Air Corps 

[sic] [Fleet], and COS South China 

Air Force. [COM-14 confused over 

whereabouts of CINC Combined 

and status of Mutsu as Flagship.] 

During last week September Mutsu

joined 2nd Flt after stint at yard at 

Kure. On 1 Oct, CINC2 “relieved as 

CINC Combined and hoisted flag 

on Mutsu.” On 17 October, CINC2 

retransmitted message originated on 

14th addressed as follows: Action: 

2ndFltColl, 3rdFltColl, Combined 

AF, SubRon5, SubRon6, AirRon7, 

BatDiv3; Info: ComCarDivs, CINC 

S 22 Sep Ashigara relieved by Isuzu as Flag of 

South China Flt. [Isuzu is Flag of 

DesRon5/3rdFlt.]

S 23 Sep Task Force being formed out of elements of 1st /2ndFlts.

S 24 Sep Additional carrier division organizing. 

[The Shokaku and Zuikaku later became 

CarDiv5.]

S 26 Sep Preparations noted for large-scale 1st/

2ndFlt exercises with carrier divisions.

S 28 Sep Preparations may indicate possible 

hostile action.

S 1 Oct 2ndBasFor [3rdFlt] to board ship 

[possibly Flagship 3rdFlt] at Sasebo 

leaving some units behind.

S 2 Oct COM-14 says, “3rdFlt being built up 

to its French Indochina composition.” 

S 4 Oct Volume of traffic since callsign change 

indicates reorganization. COM16 

agrees that flag of CINC Combined 

Flt shifted to Mutsu [BatDiv1/1stFlt] 

and Flag CINC2 is in Maya (CruDiv4, 

2nd section, 5thFlt).

M 5 Oct Carrier air groups officers told Pearl 

Harbor their objective. Training 

continued.

S 9 Oct Yokohama Air Corps addressed message to Taiwan addressee. Chitose Air 

and AirRon24 to move to Mandates 

soon. Naval Auxiliaries in Mandates 

continue to increase, now 33.

S 11 Oct Chitose Air moving to Saipan without 

commander. Yokohama Air at Truk as 

is commander AirRon24. Large air 

unit appears at Hainan and Kanoya. 

Large air movements noted in 4thFlt.

S 12 Oct Commander Yokohama air at Kwajalein.
Pearl Harbor Revisited

68

included SEF, “a circumstance reminiscent of Indochina operations when 

3rdFlt assumed major importance with 

CINC South China Force in subordinate role.”

S 24 Oct 2ndFlt sent message to unusual action 

addressees not normally under 2ndFlt 

and information addressees which 

give southern flavor to whole group. 

Action: 3rdFltColl, CombinedAirColl, 

2ndFltColl, BatDiv3, DesRon3; Info: 

Palao; Spratly/Cam Ranh; DF; Tokyo; 

CINC Combined; ComCarDiv5.

S 24 Oct Communications Officer CarDivs 

message to following addressees: BatDiv3, CruDiv8, CarDivs less CarDiv3; 

Info: Tokyo Radio, U/I addressee, 

Communications Officer Combined 

Flt. [Major units of Pearl Harbor 

Strike Force. See Appendix A, same 

date, for related messages.] 

S 26 Oct U/I Air Command to move from Taiwan to South China. 5thFlt becoming 

more tangible at Yokosuka.

S 28 Oct COS French Indochina Force aboard 

Flagship Combined Fleet. Indications 

growing that 4thFlt is preparing for 

operations. CINC4 sending traffic to 

Combined Flt, Subs, CarDivs, and 

Tokyo addressees.

S 30 Oct Tokyo ComDiv message volume 

unprecedented. Possible communications change in offing COM-14 

believes movement of 3rdFlt imminent. 

Station C reports Ashigara departed for 

South. New carrier [Koryu] associated 

with Saipan, Truk, and Yokosuka Air 

suggesting move to Saipan soon.

S 31 Oct Japanese Navy callsigns changed in 

Fleet and Air units. Commanders 11 

Combined, Radio Takao, Radio Palao, 

Radio Tokyo. A total of 38 Naval 

Auxiliaries in Mandates.

S 18 Oct CINCChinaFlt replaced; date 

unknown. Appears to be heavy air 

movement between Empire and 

Mandates.

S 19 Oct Guam reports 13 new Naval Auxiliaries in Mandates. [Hawaii and Philippines disagree over callsign for a new 

carrier—either Zuikaku or Shokaku.]

S 21 Oct Japanese DF traffic first noted on 

20 October is increasing. Stations at 

Chinkai, Manchukuo area, Chosen 

area, Jaluit, Sasebo. Combined Air 

Force sent message which included 

carriers in addresses. COM-14 notes 

this not normal and that scope of 

addees indicates a large-scale operation over a long distance. Action: Navy 

Minister, Chief Naval General Staff 

(NGS), Combined Air Force, CarDiv4 

less HOSHO and U/I, CarDiv3; Info: 

CINC China Flt, CINC South China 

Flt, all major Fleet Flagships. 

S 22 Oct 11 Maru vessels noted using suffix 

denoting “C.O. Naval Detachment 

Aboard” in Connection with Takao/

Hainan air movement. A 5thFlt continues to appear. The “Special Task 

Force” or “Southern Expeditionary 

Force” [SEF] [Flag in Kashii] associated with CarDiv4 in future operations. 

Impression grows that a large-scale 

operation is in progress in Mandates, 

in Takao/Hainan/Indochina areas and 

in Kuriles. 

S 23 Oct COM-14 noted message from DF Hq 

Tokyo to collective addressee which 

omitted CINC South China Force but 
 69

Appendix C

includes Car Divs3 and 4 plus Auxiliaries, and units of Combined Air and 

1stFlt. Uncertain area of operations.

C 8 Nov Details of U.S. aircraft in Philippines sent to Tokyo by Manila on 

1 November.

S 9 Nov COS SEF in Tokyo.

M 10 Nov Japanese Army and Navy agree to 

attack plan. Pearl Harbor Strike Force 

departs home waters for Kuriles. [See 

Appendix A.]

S 10 Nov Combined Flt mostly in Kure area, 

BatDiv3, CINC2, and two CruDivs 

noted specifically. 3rdFlt in Sasebo/

Takao; 4thFlt in Truk; 5thFlt has 

one unit at Chichi Jima; CdrCarDiv3 

possibly with Combined Flt units at 

Kure/Sasebo; CINC Combined AF at 

Takao.

C 12 Nov Messages from Tokyo to Manila on 

5 November request information regarding aircraft and ships.

S 12 Nov CarDiv3 returned to Kure from Takao 

per COM-16.

S 13 Nov Activity of BatDiv3 unclear—Flag at 

sea, Cdr in Yokosuka, DivComOfficer 

active with Truk, Saipan, Palao—other 

ships in Div unlocated. One CruDiv of 

2ndFlt in traffic with Palao, possibly in 

that area. Other units of 1stFlt seem 

inactive. Carriers inactive.

S 14 Nov 4thFlt Staff members in Tokyo. Carriers remain in home waters with most 

in port. Flag of BatDiv3 heard. DesRon3 and CruDiv7 of 2ndFlt active, 

may proceed south.

S 15 Nov Combined Fleet to BatDiv3, DesRons1/3, associated with SEF. CINC2 

and 12 Air Corps [sic], Shiogama Air 

in Takao area. COM-14 noted similarity to concentration at Hankow Air, 

July–August.

S 1 Nov All major fleet callsigns recovered. 

Shore callsigns no change. Individual 

callsigns slow to collect and recover.

S 3 Nov General messages continued to emanate from Tokyo in unprecedented 

numbers. Numbers not understood 

now that communications change 

past. COM-14 suggested that messages were reports of some kind. A new 

addressee reading 1stAirFlt noted for 

first time. This is a new organization. 

Possibly explains association between 

CarDivs3 and 4 and Combined AF, 

i.e., between shore-based and Fleet 

Air. [See Appendix A, 4 October and 

6 October, for earlier appearances of 

1stAirFlt address.]

 S 6 Nov Tokyo radio now using “general” or 

“area” calls vice unit calls and may 

have eliminated address and originator 

from messages on broadcast circuits. 

Very heavy air concentration on Taiwan includes the entire Combined Air 

Force-Commander and Staff, one carrier division, and the Fleet Air Arm. 

S 7 Nov Possible heavy concentrations in Marshalls causing congestion on Mandates 

circuits.

S 8 Nov Formation of Force in Takao/Bako 

area [Taiwan] under Cdr Combined 

Air nearly completed based on reports 

addressed to CINC Combined Flt; 

Naval Ministry; CdrCarDivs; Combined Air; 1stFlt, and Shore addressees associated with movement or 

organizational changes. Force possibly 
Pearl Harbor Revisited

70

with CINC3 continues. Partial list of 

forces being assembled by CINC2 in 

two days (27 units, 11 Marus) includes 

CarDiv3, AirRons6/7; CruDivs5/7; 

DesRons3/4/5; BasFor1/2; Shiogama Air Corps; possibly two additional air units; and 13 U/I units. 

Mandates/4thFlt: Palao seen as locale 

for forthcoming concentration of forces based on communications activity. 

Concentration in Marshalls far greater 

than Palao. 

M 22 Nov Pearl Harbor Strike Force completes 

move to Kuriles.

S 23 Nov High precedence traffic increasing. 

Typical headings are (a) from Tokyo 

Address to Collective Shore; Info: COS 

Combined, 1st/2nd/3rd/4th/5thFlts, 

SEF; (b) from COS3 to COS2; Info: 

COS Combined AF, SEF; (c) from 

Tokyo to COS3 and SEF; Info: Sama, 

Hainan; (d) from U/I Flt unit to Radio 

Takao and Hainan, U/I Flagship; Info: 

Radio Tokyo and 2ndFlt Flagship; 

and (e) from Iwakuni Air to Iwakuni 

Air Detachment at Naha, Takao; Info: 

Kure, Bako and U/I unit Takao. Indications are that 3rdFlt units are under way 

in a movement southward coordinated 

with 2ndFlt, Combined AF, and SEF.

S 24 Nov Increased activity among 3rdFlt 

addressees with a high percentage 

of movement reports. Large number of messages associate CarDiv3 

with CINC3. No definite location for 

carriers.

C 24 Nov OPNAV message warning of possible 

Japanese “aggressive movement in any 

direction.” Mentions Philippines and 

Guam as possible objectives.

most active—appears to be arranging operations of units involving 

1st/2ndFlts, carrier and air units. Purpose of air concentrated in Takao area 

unknown; possibly will move south to 

SEF.

S 16 Nov 1st/2ndFlt units remain in Kure area. 

CINC2 has assumed an important 

role involving units of several fleets, 

SEF, Combined Air, CarDivs and 

Mandates fleet. DesRon1 is operating 

with CarDivs and BatDiv 3. 

C 18 Nov OPNAV sent messages to CINCAF 

regarding Japanese patrols from Mandates and Dutch concerns/intentions 

regarding Japanese buildup in Mandates 

which also threatens Netherlands East 

Indies. 

S 18 Nov CINC Combined Flt active sending and 

receiving messages. CINC2/SEF/Combined AF association very plain in messages sent and received. BatDiv3, CarDivs, two DesRons associated in traffic. 

CINC2 in command of large Task Force 

comprising 3rd Fleet, Combined AF, 

some CarDivs, and BatDiv3. No movement from home waters has been noted. 

3rdFlt will move from Sasebo in near 

future. 2ndBasFor may be transporting 

air units or equipment.

S 20/21 Nov Unusually heavy traffic Tokyo to all 

Majcoms. NGS urgent precedence 

to COS South China Flt. Personnel 

Bureau sending long personnel messages. DF traffic heavy. Tokyo/Takao 

circuit forced into duplex to handle 

traffic. Combined Fleet: Flags of 

1st/2ndFlts in Kure area. Most of both 

Flts in Kure/Sasebo area. BatDiv3 still 

in Yokosuka area. Traffic to and from 

CINC2 abnormally high; association 
 71

Appendix C

moved to Hainan Island. It seems that 

the Second Base Force is transporting 

equipment of the Air Force to Taiwan. Radio calls for the South China 

Fleet, the French Indochina Force, and 

the naval stations at Sama, Bako, and 

Takao appear also in headings of dispatches concerning this task force. The 

Resident Naval Officer, Palao, and the 

Third Base Force at Palao have communicated extensively with the Commander-in-Chief of the Third Fleet. It 

is thought that a strong force of submarines and air groups are in the vicinity of the Marshall Islands. This force 

includes the 24th Air Squadron, at 

least one aircraft carrier, and probably 

one third of the submarine fleet. The 

14th Naval District Communications 

Intelligence Unit evaluates the foregoing information to indicate that a 

strong force may he preparing to operate in Southeastern Asia while component parts of the Task Force may operate from the Marshalls and Palao.”

M 26 Nov Pearl Harbor Strike Force departs 

Kuriles.

C 27 Nov COM-16 261331Nov41, to CINCPAC, COM-14, OPNAV, CINCAF; 

“JAPANESE NAVY-ORGANIZATION OF FLEETS Date of lssue-27 

November 1941. Following has been 

submitted by the 16th Naval District 

Communications Intelligence Unit, 

referring to and commenting on yesterday’s information from the 14th Naval 

District Communications Unit. During 

the past few days traffic analysis indicates that the Commander-in-Chief, 

Second Fleet, is directing units of the 

First, Second, and Third Fleets, and 

Submarine Force organization that 

S 25 Nov High level of traffic suggests that 

organizational arrangements or other 

preparations are not yet complete. 

Genzan Air Corps has been in Saigon 

for eight days, according to callsign 

analysis. Other units of Combined Air 

Force possibly moved to French Indochina area from Taiwan. One or more 

CarDivs in Marshalls now.

S 26 Nov CruDiv7 of Combined Fleet received 

traffic via Sama indicating arrival 

in Hainan waters, probably accompanied by DesRon3. Takao, former 

Flagship 2ndFlt active in association with 2nd/3rdFlts. No movement 

of Flags newly formed force noted as 

yet. CINC5 association with new Task 

Force.

C 26 Nov COM-14 260110Nov41 to OPNAV, 

Info: CINCPAC, CINCAF, COM-16; 

“JAPANESE NAVY ORGANIZATION OF FLEETS Submitted by the 

14th Naval District Communication 

Intelligence Unit: Since the latter part 

of October, the Commander-in-Chief 

of the Second fleet has been forming a 

Task Force consisting of Second Fleet, 

Third Fleet (including First and Second Base Forces, and First Defense 

Division), Destroyer Squadron Three, 

Submarine Squadron Five, Combined 

Air Force Air Squadron Seven. Possibly vessels of the Third Battleship 

Division in the First Fleet. Third Fleet 

units are believed to be moving in the 

direction of Takao and Bako. It appears 

that the Seventh Cruiser Division and 

the Third Destroyer Squadron are an 

advance unit and may be en route to 

South China. The Combined Air Force 

has assembled in Takao, and indications are that some of it has already 
Pearl Harbor Revisited

72

used in area operations. Commandersin-Chief, Second, Third, and Southern 

Expeditionary Fleet, appear to have 

major roles. Traffic from the Minister of 

the Navy and the Chief of Naval General Staff to Commanders-in-Chief of 

the Fleet appears normal. This evaluation is considered reliable.”

S 27 Nov Some tactical traffic from carriers 

intercepted. DF activity high. No evidence of movement Combined Fleet 

from Kure/Sasebo area. Carriers still 

located in home waters.

C 27 Nov OPNAV WAR WARNING message 

alerts all Pacific commands to “An 

aggressive move ... within the next few 

days.” Possible objectives mentioned: 

Philippines, Kra Peninsula, Thailand, 

or Borneo. Guam and Samoa to take 

measures against sabotage.

C 28 Nov Naval attaché, Shanghai, on 25 November sights troop transports heading 

southwest; on 26 November, sights 

warships led by cruiser, possibly Naka 

[2ndFlt] heading south. Also reports 

that between 19 and 26 November, 

he had sighted many transports, many 

loaded with troops, headed southwest.

S 28 Nov Communications volume between 

South China, Mandates, and Empire 

very heavy. No tactical traffic seen. 

Suspected “RI” net very active and 

becoming more so. DF and RI nets 

operating at full strength upon U.S. 

naval communications and getting 

results. In the Combined Fleet, no 

indications of any movement any Fleet 

units. In 3rdFlt, 1stBasFor possibly not 

in Sasebo but en route somewhere. In 

4thFlt area, 4thBasFor at Truk, Yokohama Air Corps at Ruotto, and Wotje 

apparently will be divided into two sections. For purposes of clarity the units 

expected to operate in South China will 

be referred to as First Section and units 

expected to operate in the Mandates 

will be referred to as Second Section. 

The estimated units in First Section are 

Cruiser Division Seven, Air Squadron 

Six, Defense Division One, Destroyer 

Squadron Three, and Submarine Squadron Six. The Second Section consists of 

Cruiser Division Five, Carrier Division 

Three (Ryujo and one Maru), Destroyer Squadrons Two and Four, Submarine Squadron Five, Destroyer Division twenty three, First Base Force of 

Third Fleet, Third Base Force at Palao, 

Fifth Base Force at Saipan, and lesser 

units unidentified. Cruiser Division 

Six and Battleship Division Three may 

be included in First and Second Sections respectively, but status cannot be 

clarified yet. There are slight indications 

today that Destroyer Squadron Three, 

Cruiser Division Seven, and Submarine 

Squadron Six are in the Takao area. The 

balance of Third Fleet units in doubt 

but may be assumed that these vessels 

including Destroyer Squadron Five will 

take stations in the Straits of Formosa 

or further south. Combined Air Force 

units from the Empire proper are in 

Paidoh. [Possibly Paiho in South Central Taiwan.] It is impossible to confirm 

the supposition of reference report that 

carriers and submarines are in the Mandates. The best indications are that all 

known First and Second Fleet carriers 

are still in Sasebo-Kure area. Directives to the above Task Forces, if such, 

are directed to individual units and not 

to complete groups. Special calls usually precede formation of Task Force 
 73

Appendix C

concentration [COM-16 disagrees]. 

Presence of AirRon24 and Yokohama 

Air suggests future Air/Sub operation from Marshalls. Presence of plane 

guard DDs suggests at least one carrier 

in Mandates.

C 1 Dec COM-16 reports ships from 3rdFlt 

arriving Formosa. Notes CINC2 in 

Flagship Atago has moved south.

S 1 Dec All radio calls of units afloat changed 

at midnight. Unusual. Service calls 

usually last six months. Suggests an 

additional progressive step in preparing 

for active operations on a large scale. 

1stFlt: nothing to indicate Fleet out of 

home waters. Believe most of 1stFlt is 

in 2ndFlt Task Force. 2ndFlt believed 

proceeding from Kure/Sasebo in direction of South China, Indochina, probably passing up Takao. CruDiv7 and 

DesRon3 definitely in Indochina area. 

3rd/4th/5thFlts NTR, Carriers, NTR; 

Combined Air Force, NTR. Large 

number of subs believed east of Yokosuka/Chichi Jima and Saipan.

S 2 Dec COM-14/COM-16 disagree on precise location of 2nd/3rdFlt Task Force. 

COM-16 places in Takao area in communications with Takao radio. COM14 did not hear but did note Takao 

sending traffic to Tokyo for these fleets 

suggesting they are not near Takao. 

Both seem to agree that the large fleet 

has left Empire waters. COM-16 

reports nine subs vicinity Cam Ranh 

Bay, possibly SubRons5/6 which have 

been included in 2ndFlt Task Force. 

Possible that Combined Fleet staff 

has split, part to 2nd/3rd Flts, part 

elsewhere. 2ndFlt believed under way 

in company. 3rdFlt NTR. Carriers, 

in communications with AirRon24 

and Kamoi.

C 29 Nov OPNAV WAR WARNING message. Text indicates Army also received 

warning. (290110Nov41)

C 29 Nov COM-16 reports CINC2 to move 

south between 29 November and 2 

December. (291029Nov 41)

S 29 Nov Traffic volume above normal. Traffic to South China very high. Intelligence-related messages numerous, e.g., 

11 from Tokyo to Majcoms. Tokyo 

Radio Intelligence sent four long messages to Majcoms. DF net very active. 

Existence of 11thAirFlt confirmed. 

Not a Navy element. Arrival of AirRon7 in Takao confirmed. Following 

units under immediate command of 

CINC2: CarDiv3, DesRon2, SubRon5, DesRon4, SubRon6, 3rdFlt, 

CruDiv5, SEF, CruDiv7, and possibly 

two battleships subordinate to 3rdFlt.

C 30 Nov OPNAV directs CINCAF to reconnoiter line from Manila to Cam Ranh 

Bay for evidence of Japanese preparations to attack Kra peninsula.

S 30 Nov Traffic volume low; old messages being 

repeated. AKAGI (CV) heard on tactical circuit. Naval General Staff sent 

one urgent precedence message to 

COS Combined, 2nd/3rd/4th/5thFlts; 

Combined AF; Subs and China Fleets. 

In the Combined Fleet, COS, Combined and 1stFlts in Kure, COS2 is 

unlocated, possibly at sea. CINC2 

addresses message to Kongo (BB) and 

Hiyei (BB) which places them in his 

Task Force. CINC3 possibly underway. 4thFlt area: CdrSubs headed for 

Marshalls. Evidence points to sub 
Pearl Harbor Revisited

74

Takao referring to the Far Eastern crisis, notes that “specific orders will be 

issued soon.” No traffic from Cdr carriers or sub force seen. In the 3rdFlt, 

a “COS” sent a message to “Commander 14th Army” aboard Ryujo 

Maru in 3rdFlt. A number of Maru 

vessels have been addressing CINC3. 

Shiogama Air and 2 U/I Corps 

are moving to probably Indochina.

C 5 Dec Diplomatic message from Manila to 

Tokyo on 28 November details U.S. air 

patrols.

C 6 Dec Diplomatic message from Honolulu 

to Tokyo on 18 November details ship 

movements in Pearl Harbor.

S 6 Dec Traffic volume heavy but much old 

traffic seen. Much confusion in routing/delivery. Four stations now holding broadcasts: Tokyo [3] Saipan, 

Ominato, and Takao. CINC4 in Truk 

area, never in Jaluit. Definite close 

association between 3rdBasFor, Palao 

and forces in South China, e.g., 2nd, 

3rdFlts, SEF, and Bako. Arrangements 

largely neglected by CINC4. 5thFlt 

dispersed in Empire waters.

almost complete blank. Traffic at low 

ebb. Not one callsign recovered. Some 

units of Combined AF have left Takao 

area. 

C 3 Dec OPNAV reports Japanese diplomats 

burning codes.

S 3 Dec 2nd/3rdFlts probably under way. Subs 

and carriers, NTR.

C 4 Dec OPNAV orders certain U.S. intercept 

sites to burn codes.

M 4 Dec Malay invasion force departs Hainan.

S 4 Dec Tokyo sends large number [12] of 

urgent precedence messages to Majcoms. Intelligence sent seven-part 

message to COS China Flt, Combined 

Fleet, 3rdFlt, South China Flt, SEF, and 

Sama. CINC2 and CINC3 very quiet 

but receiving much traffic. CINC2 in 

vicinity of Takao. Cinc Combined sent 

message to U/I; Info: 3rdBasFor, Palao, 

CINC2, and CINC3.

S 5 Dec All circuits overloaded. TokyoMandates circuit duplex. Many new 

schedules. Both Takao and Tokyo handling traffic for 2nd/3rd Flts, some of 

which is old traffic. A plaintext message 

from a ship’s captain from Tokyo to 
75 75

1. For many years communications intelligence was 

also known in the Navy as traffic intelligence if 

derived from traffic analysts, and radio intelligence

when derived from decrypted messages.

2. Since 1912, when Admiral George Dewey, chairman of the General Board, decreed that military 

men should limit themselves to “purely military 

questions,” U.S. Navy war planners did not consider 

either domestic or foreign motives for war. This 

situation prevailed until at least the 1960s. Ronald 

Spector, Professors of War: The Naval War College and 

the Development of the Naval Profession (Newport, 

RI: Naval War College Press, 1977). Cryptography 

is the development of codes and ciphers; cryptanalysis is the exploitation of codes and ciphers.

3. 18 August 1917 memorandum from Radio Officer to the Commanding Officer, Series II.E.6, 

Center for Cryptologic History (CCH) History 

Collection.

4. “Notes and Comments on the Necessity for and 

the Organization of a Cipher Bureau, 1918; A 

Summary of the Organization, Activities, and 

Achievements of the Code and Cipher Section 

of the Military Intelligence Division (M18),” 

November 1918, by Chief Yeoman H. E. Burt, 

USN, Series II.E.62 and II.E.63, CCH History 

Collection (classified).

5. The Birthday of the Naval Security Group, Special Research Histories (SRH)-150; Record 

Group 457, Records of the National Security 

Agency, National Archives, hereafter RG 457, 

NA; a series of declassified ONI memorandums 

involving Chief Radioman (CRM) H. A. Williams on 12, 15, and 16 April 1924 and 17 July 

1924 clearly show their interest in the quality of 

intercept material supplied by Station A. ONI no 

doubt would inform the Chief of Naval Operations if anything of naval or national interest was 

learned from the messages. Chief Williams was 

stationed at Shanghai at the time. Series II.E.36, 

CCH History Collection; Jeffrey M. Dorwart, 

Conflict of Duty, The U.S. Navy’s Intelligence 

Dilemma, 1919–1945 (Annapolis, MD: Naval 

Institute Press, 1983), 41–42.

6. Jack S. Holtwick, Captain, USN (Ret), unpublished manuscript, “Naval Security Group History to World War II,” SRH-355, 67, RG 457, NA; 

anecdote about Navy Slush Fund and establishment of Research Desk was taken from Captain 

Laurance F. Safford, unpublished manuscript, 

“The Undeclared War, History of RI,” SRH-305, 

RG 457, NA. For a biographic background of 

Hooper, see L. S. Howeth, Captain, USN (Ret), 

“History of Communications Electronics in 

the U.S. Navy” (Washington, DC: Government 

Printing Office, 1963), in which he is called the 

father of naval radio.

7. SRH-355, 45–48. RG 457, NA.

8. Ibid., 52.

Notes
Pearl Harbor Revisited

76

9. Dedication of the memorial to the “On the Roof 

Gang,” Series III.H.26; Interview Duane L. 

Whitlock, Captain, USN (Ret), 11 February 1983, 

by Robert Farley, OH-05-83, NSA (classified).

10. 13 May 1929 memo from CNO to CINCAF, 

Appendix A, SRH-180, RG 457, NA.

11. SRH-355, 223, footnote. RG 457, NA.

12. COM-13 correspondence with OP-20 in July 

and August 1930, Series II.H17, CCH History 

Collection (classified).

13. 22 May 1932 letter from CNO to Rear Admiral E. H. Campbell, COM-13, Series III.H.17, 

CCH History Collection (classified); see also 

SRH-355, 67, RG 457, NA.

14. 22 March 1932 memorandum to COM-13 

from CNO, Series III.H.17, CCH History 

Collection (classified) and 27 April 1932 memo 

same to same, Series III.H.17.1, CCH History 

Collection (classified); see also SRH-355, 67, 

RG 457 NA.

15. Interview John H. Gelineau, CWO, USN (Ret), 

4 October 1983, by Robert Farley, OH-22-83, 

NSA (classified).

16. 11 June 1931 memorandum CNO to COM14, Series III.H.17, CCH History Collection 

(classified). Without explaining what “unofficial 

intercept activities” meant, CNO ordered the station “reestablished.”

17. Navy records for December 1941 show the 

following dispositions: Collection–Total number of receivers, 68: Guam–9; Corregidor–25; 

Heeia–21; Bainbridge Island (formerly Astoria)–13, SRH-197 “U.S. Navy Communication 

Intelligence Organization, Liaison and Collaboration, 1941–1945,” RG 457, NA; see also SRH355, 243–44, RG 457, NA. 

18. SRH-355, 50–51, RG 457, NA.

19. Memorandum from CINC U.S. Fleet to Chief 

of Naval Operations dated 15 May 1929, Series 

III.H.1, CCH History Collection (classified).

20. 10 May 1930 letter Safford to Dyer, Series 

III.H.10.3, CCH History Collection.

21. SRH-355, 391. RG 457, NA.

22. U.S. Fleet Problems X and XI, Series III.H.10.3, 

CCH History Collection.

23. Part IV, Communications, in “Command Conclusions on U.S. Fleet Problem 9,” Series III.H.1, 

CCH History Collection (classified). This is a 

very brief paraphrase of the paragraph on radio 

security.

24. SRH-355, 155. RG 457, NA.

25. SRH-222, 223, 224, 225, RG 457, NA. Various reports on Japanese Grand Fleet maneuvers 

1930, 1933, 1934, 1935.

26. SRH-355, 131, RG 457, NA; see also Gordon 

Prange with Donald M. Goldstein and Katherine V. Dillon, The Verdict of History (New York: 

McGraw Hill, 1986), 134. On 4 November 

1941, an OPNAV message reported this fact to 

CINCPAC, CINCAF, and others. See also Safford, SRH-305, 20, RG 457, NA.

27. SRH-355, 118–119, RG 457, NA.

28. Ibid., 222–223, RG 457, NA.

29. Ibid.

30. Captain Laurance F. Safford, unpublished manuscript, “A Brief History of Communications 

Intelligence in the United States,” SRH-149, RG 

457, NA.

31. History of Signal Security Agency, Series III.hh, 

CCH History Collection (classified).
 77

Notes

Series VII, Box 4, 19, CCH History Collection 

(classified).

43. “History of JNA 20” (CORAL), Vol. I, NSA 

Cryptologic Archival Holding Area (classified).

44. SRH-305, RG 457, NA and SRH-149, RG 457, 

NA.

45. This code was stolen without the knowledge of 

the Japanese soon after its introduction in 1939. 

“History of OP-20-GYP,” 2, Series IV.W.I.5.12, 

CCH History Collection (classified). 

46. “History of OP-20-3-GYP,” 2, Series IV.W.I.5.12, 

CCH History Collection (classified).

47. Ibid.

48. Ibid.

49. “Military Study Communication Intelligence 

Research Activities,” United States Navy, 30 June 

1937, SRH-151, RG 457, NA.

50. History of OP-20-GYP-1, Series IV.W.I.5.13, 

CCH History Collection (classified).

51. SRH-149, RG 457, NA; see also 7 December 

1929 memorandum to DNI from ONC, “Radio 

Intelligence.” At that time the main purpose of 

the Research Desk was deciphering Japanese 

codes, Series III.G.9, CCH History Collection; 

Japanese traffic volumes, Howe monograph, Early Background of the U.S. Cryptologic Community, 

Series VII.15, CCH History Collection; ArmyNavy DOE, 1931, Navy proposed to divide diplomatic communications based on availability of 

intercept and Naval power, Series VII.19, Box 4, 

Vol. I pre-1942, CCH History Collection (classified); Japanese Blue Book introduced February 

1931, SRH-305, RG 457, NA.

52. “History of OP-20-GYP-1,” Series IV.W.I.5.13, 

CCH History Collection. See also Edwin T. 

Layton, And I Was There (New York: William 

Morrow, 1985), 77.

32. Herbert O. Yardley, The American Black Chamber, 

originally published in book form 1 June 1931.

33. 29 October 1931 memorandum from DNC 

to CNO via DNI, “Allocation of RI Activities 

Between the Army and the Navy,” Series VII.19, 

Box 4, Vol. 1, pre-1942, CCH History Collection 

(classified).

34. 29 October 1931 memorandum from OP-20-G 

to CNO via DNI, Series VII.19, Box 4, Vol. 1, 

CCH History Collection (classified).

35. 12 April 1933 memorandum for the Director of 

Naval Communications from J. W. McClaran, 

OP-20-G, Series VII.19, Box 4, Vol. 1, CCH 

History Collection (classified).

36. There is no record of this episode in the Department of State files.

37. RG 220, NA. Proceedings of the Joint ArmyNavy Board, National Archives, Joint Board 

Memorandum, 24 April 1933.

38. A series of unclassified memoranda between 

CNO, the Joint Army-Navy Board, and the 

Secretary of War during the period March–July 

1933, Joint Board #319, Serial 516, RG 220, 

NA.

39. Prescott H. Currier, Captain USN (Ret), interview by Robert Farley and Henry Schorreck, 14 

November 1980, Oral History 39-80 (classified), CCH.

40. SRH-149; SRH-305, RG 457, NA.

41. OP-20-G memorandum Serial 051220 dated 

25 July 1940, “Coordination of Intercept and 

Decrypting Activities of the Army and Navy,” 

Series VII.18, Box 4, 19, CCH History Collection (classified).

42. See 14 February 1946 memorandum for OP-20-

4 from Captain L. F. Safford, “Responsibility for 

Decoding and Translating Japanese Intercepts,” 
Pearl Harbor Revisited

78

light cruisers and destroyers, 10.10.7; and submarines, 1.1.1. See also Prange, Verdict of History, 

7. By standing aloof from the League of Nations, 

the United States had rejected collective security, 

yet it had refused to provide for itself an adequate 

unilateral national defense system. Prange also 

asserts a weak argument supported by several 

quotations that the blame for Pearl Harbor actually lay with the American people who, because 

of complacency, idealism, and a false sense of 

security based on racial arguments, failed to 

insist that Congress provide for adequate military strength after World War I.

65. “U.S. Naval Pre-World War II Radio Intelligence 

Activities in the Philippine Islands, 1931–1942,” 

SRH-180, RG 457, NA.

66. SRH-355, 421, and Appendix VII, 289–90. The 

linguist in at least one instance was also head of 

the Diplomatic section in Station C.

67. SRH-355, 311, RG 457, NA. 

68. Interview, Thomas H. Dyer, Captain, USN 

(Ret), January–March 1952, by Robert Farley, OH-01-82, CCH, and NSA interview 

Rudolph T. Fabian, Captain, USN (Ret), 

4 May 1983 by Robert Farley OH-09-83, 

CCH (classified). Hereafter “Dyer interview,” 

“Fabian interview”.

69. SRH-355, 167–168, RG 457, NA.

70. Dyer interview.

71. Dyer interview; interview Wesley A. Wright, 

Captain, USN (Ret), 24 May 1982, by Robert 

Farley and Henry Schorreck, OH-11-82, CCH; 

interview Edwin T. Layton, February 1983, by 

Robert Farley, OH-02-83, CCH.

72. SRH-355, 330–331, RG 457, NA.

73. Ibid., 218–222, RG 457, NA.

74. Military Study, SRH-151, RG 457, NA.

53. “History of OP-20-GYP-1,” Series IV.W.I.5.13, 

CCH History Collection (classified).

54. Ibid.

55. Ibid.; see also Layton, And I Was There, 77–78, 

249.

56. “History of OP-20-GYP-1,” Series IV.W.I.5.13, 

CCH History Collection (classified).

57. Interview Frank B. Rowlett, 26 November 1974, 

by Vincent Wilson, Henry Schorreck, David 

Goodman, and Earl Coates, OH-01-74, NSA 

(classified).

58. Ibid.

59. Friedman monograph Preliminary Historical 

Report on Solution of the B Machine, 14 October 

1940, Series IV.I.2.28, CCH History Collection 

(classified).

60. SRH-305, RG 457, NA.

61. Part of OP-20-G’s workload is described in 

SRH-149, RG 457, NA and SRH-305, RG 

457, NA. See also the history of OP-20-GY, 

Series IV.W.I.5.10, CCH History Collection 

(classified).

62. History of Signal Security Agency, Vol. III (classified), contains a copy of appropriate portions 

of the act, Series III.hh. See also W. J. Holmes, 

Double-Edged Secrets (New York: Berkley Books, 

1981), 13–14, which states that this act prohibited a commercial radio company in Honolulu 

in early 1941 from sharing daily Japanese ship 

position data inherent in commercial weather 

messages with COM-14’s intelligence collection 

effort.

63. Prange, Verdict of History, 7.

64. H. P. Willmott, Empires in the Balance (Annapolis, MD: Naval Institute Press, 1992), 30–51. The 

actual ratios: battleships and heavy cruisers, 5.5.3; 
 79

Notes

80. Ronald Spector, Professors of War: The Naval War 

College and the Development of the Naval Profession (Newport, RI: Naval War College Press, 

1977), 71–72, 108–111.

81. Both Laurance Safford and A/CNO RADM 

Royal H. Ingersoll indicated (in Pearl Harbor 

Attack Hearings Before the Joint Committee 

on the Investigation of the Pearl Harbor Attack, 

Congress of the United States, 79th Congress, 

hereafter PHA, part 26, 388 and part 9, 4241, 

respectively) in their testimonies that they 

believed Hawaii was responsible to CINCPAC; 

Safford in his testimony to the Hart inquiry. 

Ingersoll while being cross-examined by Kimmel 

during the congressional investigation. It was 

Safford, of course, who in 1937 opened the office, 

assigned Dyer and others to COM-14, and allocated all cryptanalytic tasks until 1942.

82. Memorandum from CNO to COM.14, 21 

August 1937, “Cryptanalysis, Orange M1 System” Series III.H.17, CCH History Collection 

(classified). The M-1 was a Kana cipher machine 

used by the Japanese Navy from July 1933 for 

about four years. It is described in RIP28, 28A, 

41A, and 41B. Jack S. Holtwick, A Guide to Foreign Cryptographic Systems: U.S. Navy Short Titles, 

Cover Names, and Nicknames, 14 June 1971. NSA 

Cryptologic Archival Holding Area (classified).

83. 14 February 1938 memorandum CNO to 

COM-14, “Hawaiian Decrypting Unit, Equipment and Personnel for,” Series III.H.17, CCH 

History Collection (classified).

84. Prange, Verdict of History, 212. CNO Admiral 

Stark was committed to the Germany-first strategy, and he hoped to ward off conflict with Japan 

as long as possible. 

85. SRH-355, 368–370. RG 457, NA. See also 

Prange, Verdict of History, 400, where Bloch is 

shown as not understanding his COMINT 

unit as a source of information on Japanese fleet 

activities when he told General Short that such 

75. Asiatic (1), Mid Pacific (2), West Coast (3). 

Cavite–18*, Heeia–17*, Astoria–7, Guam–10*, 

Shanghai–7, Augusta–1 (*includes 2 for HFDF). 

East Coast-U.S. Fleet (4), East-West Coast 

DF (5), Winter Harbor–7, CINCUS–10, Navy 

Dept.–2, Students–8, Total 87. “A Military Study 

of the Radio Intelligence Organization,” June 

1937, Series IV.W.X.11, CCH History Collection (classified).

76. Cavite, a minor research center, 6; Flagship 

Asiatic Fleet, an advanced mobile unit, 3; Flagship U.S. Fleet, a mobile unit, 4; Pearl Harbor, a 

major research unit, 5; and Washington, a major 

research unit, 25, including administrative personnel. Ibid.

77. On 3 July 1940, the COMINT unit aboard the 

USS Trenton was transferred to the USS Omaha in Squadron 40T and began covering Italian 

naval circuits in the Mediterranean. Naval battles 

between Italian and British forces during July 

and August and the attack on Rhodes in September 1940 were reported by this unit. SRH355, 372, RG 457, NA.

78. The material in this section concerning U.S. 

strategic planning is based on several sources 

“War Plan Orange: Evolution of Strategy,” Lewis Morton, World Politics, January 1959, 221–250; 

“Naval Contingency War Plans 1891–1945,” 

Scholarly Resources, Wilmington, DE, 1979; “The 

U.S. Navy: Strategy, Defense and Foreign Policy 1932–1941,” Michael Kedian Doyle, Ph.D. 

dissertation, University of Washington, 1977, 

University Microfilm International, Ann Arbor,

MI, and London; “The U.S. Navy and War Plan 

Orange 1933–1940: Making Necessity a Virtue,” 

Michael K. Doyle, Naval War College Review, 

May–June 1980; and “Strategic Planning for 

Coalition Warfare 1941–1942,” Maurice Matloff and Edwin M. Snell, Office of the Chief of 

Military History, Dept. of the Army, Washington, DC, 1953.

79. Ibid.
Pearl Harbor Revisited

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97. SRH-355, 316–17, 394, 351, RG 457, NA. A 

letter from Nimitz to Halsey on 28 Oct 1942 

indicates that even at that date, motorcycles were 

still used for carrying traffic from Wahiawa to 

Pearl Harbor, Series IV.Q.5.5, CCH History 

Collection.

98. Samuel Eliot Morison, History of U.S. Navy in 

World War II (Boston: Little Brown, 1975), Vol. 

III, 42–43; NSA interview, Thomas H. Dyer, 

OH-01-82; Edwin T. Layton, And I Was There, 51.

99. See Appendix C in toto and “Combat Intelligence Unit, 14th Naval District,” TI Summaries with comments by CINCPAC War Plans 

and Fleet Intelligence.” SRMN-012, RG 457, 

NA. See also Prange, Verdict of History, 453–63, 

which cites Kimmel’s testimony at the congressional hearings when he said he had no reason 

to suspect the six missing carriers had been converted into a “lost fleet” during November. In 

fact, before the 17 November procedure change 

it was clear that these carriers had been assigned 

to a separate organization, the 1st Air Fleet. (See 

Appendix A.) However, neither COM-16 nor 

Layton picked this up as an important development though it was reported by COM-14 on 

6 November. See Appendix C. See also “Notes 

on Communications Security,” prepared by J. K. 

Dennis, 17 November 1942, Series IV.W.I.5.8, 

CCH History Collection (classified).

100. Intelligence Reports by Pacific Fleet Intelligence 

Officer, PHA, part 17, 2643.

101. See Appendix C and SRMN-012: COM-14 

Daily Comint Summaries for 16, 31 July; 28 

September; 2, 16, 21, 22, 23 October; 6, 21, 29 

November; and 2 December. See also footnote 

1 in Prange, Verdict of History, 446, which refers 

to Kimmel’s testimony before Congress (Part 6 

beginning on p. 249). Clearly reflecting Layton’s 

assessment of traffic analysis and D/F as sources, Kimmel describes information thus derived 

as “open to serious doubts” unless supported by

readable messages. Examples cited show how 

information came from Washington. In addition, Prange also attributes to Captain Ellis M. 

Zacharias, USN, ONI, that Bloch was antagonistic toward intelligence (possibly just toward 

Zacharias) and that Kimmel should have either 

replaced Bloch or corrected the problem.

86. SRH-355, 370. RG 457, NA.

87. SRH-355, 402. RG 457, NA.

88. Radio Intelligence Publications (RIP) No. 39 

and 51. SRMN 069 and 072. RG 457, NA.

89. SRH-355, 223, 248, 260 RG 457, NA; NSA 

interview David Snyder, CWO, USN (Ret), 

3 October 1983 by Robert Farley, OH-21-83 

(classified); “History of OP-20-GYP-1” Series 

W.I.5.13, CCH History Collection (classified).

90. See also SRH-180. RG 457, NA. US. Naval PreWWII RI Activities series.

91. Appendix A, Study of Intercept Activities 23 

Aug 1940, Revised 27 September 1940, Series 

VII, Box 4, Vol. I, pre-1942, CCH History Collection (classified).

92. L. S. Howeth, History of Communications Electronics in the U.S. Navy (Washington, DC: Government Printing Office, 1963), 543.

93. “History of OP-20-GYP-1, Series IV.W.I.5.13, 

CCH History Collection (classified), and SRH355, 420. RG 457, NA.

94. David Snyder interview (classified); Prescott 

Currier interview (classified). SRH-355, 420–21. 

RG 457, NA.

95. SRH-355, 420-21. RG 457,NA.

96. NSA interview, Thomas H. Dyer. Captain USN 

(Ret), Jan–Mar 1982 by Robert Farley, OH-01-

82 CCH.
 81

Notes

from Corregidor. The abundant detail of these 

reports contrasts sharply with the histories written of the period such as Morison’s Vol. III, Rising Sun in the Pacific, because historians such as 

Morison did not have access to this type of record. 

The purpose of preparing Appendix C is to show 

the value of COMINT as a source of indications, 

intentions, order of battle, and warning, even when 

the underlying messages cannot be exploited.

112. OP-20-GT-P, the traffic analysis section in 

Washington, was not established until early 1942.

113. Ibid.

114. Gordon Prange, At Dawn We Slept (New York: 

McGraw-Hill, 1981), 326–33; on or about 5 

November 1941. Combined Fleet Operations 

Order #1, revealed Pearl Harbor as the Strike 

Force objective; SRN-117687, RG 457, NA, this 

message imposes radio silence on 6th Fleet submarines on 11 November 1941; SRN-1 16866, 

RG 457, NA, this message shows that radio 

silence was imposed on the entire Combined 

Fleet on 26 November 1941.

115. See Appendix A. See also Prange, At Dawn 

We Slept, chapter 40, 320–25, and Verdict of History, chapter 25, which discuss Japanese efforts to 

modify torpedoes and Kimmel’s conviction that 

torpedoes could not run in the shallow waters of 

Pearl Harbor after being launched from a plane. 

Had he seen these messages he would no doubt 

have changed his mind.

116. See message of 4 November 1941, Appendix A.

117. SRN-117453, 116476, 117301, and 116323, 

RG 457, NA. See also Prange, At Dawn We 

Slept, chapter 40, 320–25.

118. SRN-117453, 116684, 117301,116323,117665, 

115709, 115784, RG 457, NA.

119. SRN-116239/1116901, 115709, 116588, 

116131, 116583, RG 457, NA.

closely he was following T/A reports from both 

COM-14 and COM-16, however.

102. Whitlock interview (classified) and Appendix C.

103. P.H.A. Part 14, 1405, item 33.

104. Considering that Station C belonged to the 

16th Naval District, it may seem a bit unusual 

for CNO to use the CINCAF channel without 

reference to COM-16. The 16th Naval District, 

however, was a problem district/command. It was 

subordinate to CINCAF, but after 1939 the Navy 

had trouble keeping the commandant’s position 

filled. Between 1939 and 1941, three men, Rear 

Admiral John M. Smeallie, Rear Admiral Harold 

M. Bemis, and Rear Admiral Francis W. Rockwell, were appointed. It was not uncommon, 

as the Japanese crisis grew more intense, to see 

communications such as this concerning Station 

C passing directly from Washington to Hart, 

bypassing COM-l6 entirely. 

105. Morison, Vol. III, 71.

106. John B. Lundstrom, The First South Pacific Campaign; Pacific Fleet Strategy December 1941–June 

1942 (Annapolis, MD: Naval Institute Press), 8–11. 

See also Mitsuo Fuchida and Masatake Okumiya, 

Midway, the Battle that Doomed Japan (Annapolis, 

MD: U.S. Naval Institute, 1955), 48 and following. 

107. Ibid.

108. H. P. Willmott, Empire in the Balance (Annapolis, MD: Naval Institute Press, 1982), 80–82.

109. Morison, Vol. III, 161.

110. Morison, Vol. III, 87.

111. Appendix C concerns the movements of Japanese navy, air and military units of the Southern 

forces. Its contents were extracted from daily 

reports between July and December 1941 prepared by the COMINT unit in Hawaii, which 

drew on intercept from Heeia as well as reports 
Pearl Harbor Revisited

82

134. See Wohlstetter, 221 and 227, for example, and see 

Appendix B. See in particular the message translated on 4 December from Rome to Washington.

135. Ibid.

136. Connorton, Vol. I.

137. See Appendix A.

138. James Leutze, A Different Kind of Victory (Annapolis, MD: Naval Institute Press, 1981), 198–230.

139. See Appendix C.

140. Morison, Vol. III, 151.

141. Leutze, A Different Kind of Victory, 226.

142. Thomas C. Hart, Narrative of Events, Asiatic 

Fleet, a personal diary (Washington, DC: Naval 

Historical Archives).

143. Leutze, A Different Kind of Victory, 226.

144. Hart, Narrative of Events, Asiatic Fleet.

145. SRN-117284, 116772, 117290, RG 457, NA.

146. SRN-116645, 116647, 116729/1116730, RG 

457, NA.

147. Leutze, A Different Kind of Victory, 222, and 

Commander Walter Karig and Lieutenant Welbourne Kelley, Battle Report, Pearl Harbor to 

Coral Sea (New York: Farra and Rinehart, 1944), 

129–130.

148. SRN-117284, 1167712, 117290, RG 457, NA.

149. Tentative chronology/memorandum for the 

record dated 3 August 1944 from OP-20-C, 

NMCG, subject: “Assignment of Washington 

on Various Naval Systems during the Year 1941,” 

found in Rochefort papers, Series IV.W.X.1.a, 

CCH History Collection (classified).

150. OP-20-G was completely reorganized in February 1942, though not along the lines recommended by Safford. On 5 March 1942, COM-16 

was directed by CINC to “evacuate personnel of 

Radio Intelligence unit as soon as possible.”

120. Morison, Vol. III, 89.

121. Ibid.

122. SRN-115787, RG 457, NA.

123. SRN-115375, 117683. RG 457, NA.

124. SRN-115787. RG 457, NA.

125. SRN-117673, 117674/117666, 116990/116329, 

116436, 116643, 116920, RG 457, NA. See also 

Prange, At Dawn We Slept, 342–52.

126. SRN-116588, RG 457, NA.

127. See Appendix A.

128. See Appendix A.

129. Connorton, Vol. I.

130. See Appendix C, where all the OPNAV warning messages of November 1941 are shown in the 

specific context of the COMINT summaries from 

Hawaii and indirectly from Corregidor. See also 

PHA, part 10, 4834, in which Layton asserts his 

conviction that the two messages in fact stimulated the war warning from CNO on 29 November. 

131. See Appendix B. In preparing Appendix B, I 

have selected certain Japanese diplomatic messages intercepted and translated between July 

and 6 December 1941. The messages were usually seen by authorized recipients on the day they 

were translated. Army and Navy translations are 

shown in juxtaposition with the warning messages issued by the Chief of Naval Operations 

using the heading OPNAV.

132. Roberta Wohlstetter, Pearl Harbor, Warning 

and Decision (Stanford, CA: Stanford University 

Press, 1962).

133. Wohlstetter, 176–86.
83 83

CL Light Cruiser

CNO Chief of Naval Operations, OPNAV

Coll Collective

COM-16 Commandant, 16th Naval District

ComCarDiv Commander, Carrier Division

COMINT Communications Intelligence

COMSEC Communications Security

ComUnit Communications Unit

COS Chief of Staff

COS2 Chief of Staff, Second Fleet

COS Combined Chief of Staff Com- 

 bined Fleet

CruDiv Cruiser Division

CV Fleet Carrier

CVL Light Carrier

DD Destroyer

DesRon Destroyer Squadron

DF Direction Finding

DNC Director of Naval Communications 

 (OP-20)

DNI Director of Naval Intelligence 

 (OP-16)

1st AirFltStf First Air Fleet Staff

1stSecCh First Section Chief

6FLT Sixth Fleet 

Addees Addresses 

Adm Admiral

AirRon Air Squadron

AJCNO Assistant Chief of Naval Operations

ALNAV All Navy 

BatDiv Battleship Division

BB Battleship

BasFor Base Force

Bdcst Broadcast 

CA Heavy Cruiser

C/A Cryptanalysis

CarDiv Carrier Division

Cdr Commander

CI code instruction

CINC Commander in Chief

CINC2 Commander in Chief, Second Fleet

CINCAF Commander in Chief, Asiatic Fleet

CINCPAC Commander in Chief, Pacific Fleet

Abbreviations Used
Pearl Harbor Revisited

84

PHA Pearl Harbor attack congressional 

 hearings

R/Adm Rear Admiral 

RCA Radio Corporation of America

Reg’t Grp Regimental Group

RG 457, NA The Birthday of the Naval Security 

 Group SRH-150; Record Group 

 457, Records of the National Secur-

 ity Agency, National Archives 

RI Radio Intelligence

RIP Registered Intelligence Publication

RNO Resident Naval Officer

Scty Secretary

SECNAV Secretary of the Navy

SEF Southern Expeditionary Force

SIGINT Signals Intelligence 

SRH Special Research Histories

SS Submarine

T/A Traffic Analysis

Tech Bur Technical Bureau

Tfc Traffic

T/I Traffic Intelligence

Tokyo Comms Tokyo Communications Unit

UK United Kingdom

VAdm Vice Admiral

Yrd Yard

DOE Division of Effort

FECB Far East Combined Bureau

Flt Fleet

GC&CS Government Code and Cipher School

GD Guard District

GdDiv Guard Division

Hydro Hydrographic

InfBde Infantry Brigade

InfDiv Infantry Division

Majcom Major Command

MI Military Intelligence 

Mil Prep Military Preparations

Msgs Messages

NavalAux Naval Auxiliaries

NavalSec Naval Section

ND Naval District

NGS Naval General Staff

NTR Nothing to report

OIC Officer in Charge

ONI Office of Naval Intelligence

OP-20-G Office of Chief of Naval Opertions, 20th Division of the Office 

of Naval Communications, G Section/Communications Security

OPNAV Chief of Naval Operations

Orange Japan

OTRG On the Roof Gang

PBY Amphibious Patrol aircraft

P.H. Pearl Harbor
85 85

courtesy and his enthusiastic support in obtaining 

records of OP-20-G from Crane; to the personnel at the Naval Academy Library for their help in 

locating obscure works in their extensive collection 

on the history of naval planning and access to their 

invaluable microfiche records of the Orange Rainbow plans; and to the assistance of the historian 

at the Classified Naval Archives for alerting mc to 

the Hart diary and to the records of Orange Rainbow and other prewar planning initiatives taken by 

the U.S. Navy.

Of the million or more pages of documentation supplied the National Archives by NSA, I 

have drawn extensively on the following series as 

they pertain to Japanese matters: the SRH series 

containing narrative materials pertaining to cryptologic history; the SRN series, which consists of 

individual translations of Japanese Navy messages; 

and the SRMN series, which represents discrete 

records of historical cryptologic impact originated 

by the U.S. Navy. All of this material can be found 

in RG 457.

Within the body of these records, after the translations of Japanese Navy messages (SRN series), two 

publications stand above all others: SRH-012, John 

V. Connorton’s effort on Japanese diplomatic messages, which is Volume I of his monumental work 

entitled “The Role of Radio Intelligence in the 

American-Japanese Naval War,” published in 1943; 

and SRMN-012, the Combat Intelligence Unit, 

Primary Sources

The NSA History Collection consists of manuscripts, memoranda, studies, and interviews 

related directly or indirectly to the cryptologic history of the United States. The extensive records in 

Series III (1919–39); Series IV, pertaining to the 

years of World War II; and Series VII, a special 

series, upon which I have drawn for my research 

were collected by former NSA Historian Henry F. 

Schorreck.

Also included within the files of the history program are important special collections of personal 

papers including those of William F. Friedman and 

Carter W. Clarke. These collections, however, have 

remained intact apart from the index system and 

have their own finding aids. 

Other archival collections which have also 

proved invaluable are the National Security Agency Cryptologic Archives, Ft. George Meade, MD; 

the Classified Naval Archives, Washington Navy 

Yard, Washington, DC; the Roosevelt Library, 

Hyde Park, NY; the Naval Security Group Repository, Federal Records Center, Naval Weapons 

Supply Center, Crane, IN; the U.S. Naval Academy Library; and of course the National Archives, 

Washington, DC, which houses Record Group 

(RG) 457, the NSA collection. I would like to 

make special acknowledgment to Brian von Swearingen, the Naval Security Group Historian, for his 

Notes on Sources
Pearl Harbor Revisited

86

Jr.; Jeffrey Dorwart; and James Leutze, the biographer of Admiral Thomas C. Hart, to name but a 

few, I also benefited immensely from the work of 

two English authors, H. P. Willmott and Christopher Thorne. Their books, Empires in the Balance 

and Allies of a Kind, respectively, provided profound 

commentary on Japanese motives, aspirations, and 

planning.

Two other secondary sources deserve separate 

and special mention. Almost before the fires were 

extinguished at Pearl Harbor, the executive branch 

of the U.S. government, as well as the Navy and the 

Army, had launched investigations into this terrible 

disaster. All of the reports generated by this activity were consolidated into a single, massive, thirtynine-volume (plus appendixes) report of the 79th 

Congress entitled “Pearl Harbor Attack, Hearings 

before the Joint Committee on the Investigation 

of the Pearl Harbor Attack.” Cited herein as PHA, 

such material was most valuable in preparing this 

history. For the period before the war, I am equally 

indebted to an unpublished manuscript prepared 

by the late Jack S. Holtwick, Captain, USN (Ret), 

entitled “Naval Security Group History to World 

War II.” This manuscript, which has been turned 

over to the National Archives, would have been 

listed as a primary source if all the documents 

uncovered by Holtwick could be examined by other historians. 

Two sources I could not locate would have 

added significantly to this history: Corregidor’s 

records, which were probably burned; and records 

from Washington that discuss the relationship 

between OP-20-G, ONI, and War Plans during 

the days before Pearl Harbor. Despite their obvious value and importance to this story, I doubt if 

anything will ever again match the satisfaction 

of finding in Gordon Prange’s book At Dawn We 

Slept the name of Lieutenant Commander Suguru 

Suzuki, who had been a Japanese spy at Pearl Harbor until early November 1941. The satisfaction 

came because I had already encountered his name 

14th Naval District Traffic Intelligence Summaries, 

published daily after 16 July 1941, with comments 

by CINCPAC Fleet Intelligence and CINCPAC 

War Plans. 

Finally, it should be apparent that some of 

my material concerning cryptologic operations in 

Hawaii and the Philippines has been drawn from 

personal experiences. For this I am indebted to the 

NSA Oral History (OH) program administered by 

the late Mr. Robert Farley. Copies of all interviews 

cited are located at NSA.

Secondary Sources

The noncryptologic elements of this chapter 

in COMINT history necessarily drew on 

the perspective of many others, from diarists to 

distinguished historians, to reconstruct plausible 

cause-and-effect relationships between historical 

and cryptologic developments. Since the result is, 

I believe, a unique view, particularly of the final 

months of 1941, made possible by heretofore unexamined material, I must take full responsibility for 

its conclusions.

Two of the military service official histories were 

very useful. The official Navy history was of significant and continuing value. Samuel Eliot Morison’s 

History of U.S. Naval Operations in World War II provided invaluable perspective on world disarmament 

and Japanese relations with China, as well as precise 

details on Japanese Navy and Army order of battle 

in the western Pacific on 7–8 December 1941. Of 

necessarily lesser importance in what is essentially 

a Navy-oriented history, but still valuable, is Louis 

Morton’s treatment of Japanese preparations for war 

and opening strategy in The U.S. Army in World War 

II, The War in the Pacific, and Strategy and Command: 

The First Two Years, which also provided the inspiration for some of my illustrations.

In addition to many fine American authors 

such as Gordon Prange; Edwin Layton; Clay Blair, 
 87

Notes on Sources

their encouragement, guidance, and timely criticism 

in the construction of this history. They overcame 

my initial misgivings about preparing still another 

history of this period and convinced me that the 

Agency’s archives contained unique and undiscovered treasures. 

in messages on 18 and 19 November 1941, as he 

was being transported to Hittokapu Bay probably 

to deliver his report.

I would like to thank all the members of the 

former History and Publications Division, particularly Henry F. Schorreck and Gerald K. Haines, for 
88

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