Series V 

Volume4 

National 

Security Agency 

Central 

Security Service 

CCH-N63-00-02 

So Power Can Be 

Brought into Play: 

SIG INT 

and the Pusan Perimeter 

- . 

,t 
Cover Photo: 

U.S. Marines in Pusan, 2 August 1950 

(Courtesy of the Department of the Army) 
UNITED STATES CRYPTOLOGIC HISTORY 

Series V 

The Early Postwar Period, 1945-1952 

Volume 4 

So Power Can Be Brought into Pl 

SIGINT and the Pu an Peritnct · 

Jill Frahm 

CENTER FOR CRYPTOLOGIC HISTORY 

NATIONAL SECURITY AGENCY 

2000 

INTRODUCTION 

In August 1950, the war in Korea was not going well for General 

Walton H. Walker and the U.S. 8th Army. The inexperienced 

soldiers were trapped by No1th Koreans in a small corner of South 

Korea known in the press as the Pusan Perimeter. Supplies were 

limited and morale was very low. In this desperate time, General 

Walker had a weapon - a secret weapon - that would help save the 

day: signals intelligence (SIG INT) from the Armed Forces Security 

Agency (AFSA) and the service cryptologic organizations. 

General Walton H. Walker, left, with General MacArthur 

In his book The Forgotten War, Clay Blair states that "As a 

result of the effort directed toward Moscow and elsewhere, North 

Korean codes had of necessity been grossly neglected" by AFSA. 

Further, General Walker "had no flow of 'strategic' enemy radio 

traffic from Washington to assist him. What he got he had to 
acquire locally .... " 1 In truth, AFSA was a major contributor to the 

intelligence on North Korean operations, providing, among other 

things, the "flow of strategic enemy radio traffic" 2 that probably 

saved Walker and the 8th Army on the Pusan Perimeter. Although 

it is true that N01th Korean communications were given only token 

attention before the war, talented AFSA analysts quickly started 

studying them after the initial assault, delivering intelligence to 

customers in a matter of a few weeks. Overcoming budget and 

personnel shortages, AFSA provided valuable information to 

numerous entities, including the Army Security Agency Pacific 

(ASAP AC) and American troops in the field, that would have 

otherwise been unavailable. With few other intelligence sources 

relating to North Korea available, SIGINT proved vital to the U.S. 

military efforts in the first months and throughout the war. 

THE CRYPTOLOGIC WORLD 

AFTER WORLD WAR II 

The U.S. SIGINT world after World War II was one of 

departing personnel and budget cuts. At the end of the war, United 

States military personnel desperately wanted to return to civilian 

life and their families. They deluged American newspapers and 

their congressmen with pleas to be demobilized quickly so they 

could return to a "normal" life. Their successful campaign led 

Congress to drastically reduce the size of the U.S. armed forces over 

the next few years. The army, for example, went from over 12.1 

million personnel in August 1945 to somewhat over one million by 

July 1947.3 All SIGINT operations suffered from this significant 

loss of personnel, especially those that relied heavily on men and 

women in uniform. Even though stellar analysts like Solomon 

Kullback and Frank Rowlett remained, a large number of 

personnel left cryptology at the end of World War II. 

This loss was made even more significant when combined with 

the drastic budget cuts that occurred at about the same time. There 

was no money to replace the individuals who had left the SIGINT 

world for civilian life. From 1945 until the start of the Korean War, 

President Truman slashed the U.S. military budget to help to pay 

2 
off the deficit caused by the recently ended war and to finance 

foreign aid promised through the Marshall Plan. Due to this and 

the personnel cuts, SIG INT activities were drastically pared back to 

the minimum requirements; only the most critical needs were 

filled. These factors contributed, in part, to the formation of AFSA 

in May 1949 as an effort to survive the lean times by combining 

reduced SIGINT resources. 

In the five years before the start of the Korean War, the focus of 

United States SIGINT was on the Soviet and Communist Chinese 

problems. This decision was logical due to the increasing tensions 

of the Cold War and the fall of China to the communists. In order 

to provide adequate resources for these problems, targets of 

apparently lesser concern, like North Korea, were given less 

attention. 

The allocation of the limited intercept and analytic resources 

was determined by a priority list. The Recurrent Intelligence 

Requirements List was the main SIGINT priority list, ranking 

targets in three separate groupings; the "A" list contained the 

highest priority targets, and the "C" list detailed items of lower 

interest. Recurrent Intelligence Requirements List No. 2 was 

published on 15 December 1949 and was in effect until 15 May 

1950. Of the twenty items on the "A" list, North Korea appeared 

only once in the entry: "Soviet activities in North Korea." There 

were two items related to North Korea among the fifty-eight items 

on the "B" list: "North Korean - Chinese Communist relations" 

and "North Korean - South Korean relations, including actions of 

armed units in border areas." Of the eighteen items on the "C" list, 

only "Views on U.S. Policy in Japan and Korea" dealt with either 

Korea at all. No reference was made to North Korea on any of the 

subsequent supplements to the Recurrent Intelligence 

Requirements List No. 2. Recurrent Intelligence Requirements 

List No. 3, dated 15 May 1950, dropped all mention of either Korea 

from both the "A" and "C" lists, but retained the two Korean items 

that appeared on the "B" list in Recurrent Intelligence 

Requirements List No. 2. 

Although North Korea appeared in both List No. 2 and List No. 

3, there was still almost no coverage. Intercept requirements lists 

3 
such as these were best-case scenarios. Given the conditions prior 

to the Korean War, such requirements were almost never 

completely met in reality. AFSA simply had to cover what targets it 

could and slight the rest. 

And so, the North Korean target was ignored, and no intercept 

site collected North Korean signals until 1949. Collection began 

inadvertently in April 1949 when a U.S. Army intercept station 

began collecting communications sent on an unidentified net using 

Soviet procedures. After some analysis in the field, the material 

was tentatively identified through direction finding as North 

Korean. In April 1950, the collected data were forwarded to AFSA 

for further analysis. After some study, AFSA analysts concurred 

that the intercept was North Korean. Dedicated North Korean 

collection was initiated on 21 April 1950 when, at the request of G2 

(Intelligence Staff), AFSA tasked an intercept position in Japan to 

search for North Korean communications. A second position was 

added sometime before the end of June, and approximately 200 

messages were collected before the war began. While the two 

intercept positions dedicated to North Korea took away from the 

higher priority Chinese and Russian targets, the loss was deemed 

necessary so AFSA could obtain the minimum coverage essential to 

develop a knowledge of North Korean communications for future 

COMINT needs. 

AFSA was unprepared for the war with North Korea on the 

analytic front as well. In June 1950, there was one traffic analyst 

working on North Korean communications part-time, one parttime cryptanalyst, and one Korean linguist. The linguist was a 

senior Japanese linguist who had been studying Korean in his 

spare time since July 1949. It is unknown how far his Korean 

studies had progressed by June 1950. Preliminary work on the 

small number of North Korean messages available indicated that 

there were probably two networks in use, likely carrying North 

Korean military and North Korean police communications. 

However, the low volume of traffic, the little time the analysts could 

spend studying the North Korean messages, and the lack of 

linguists available to assist in the work severely hampered the 

production of intelligence from these communications. The limited 

effort was further hindered by the fact that there were no Korean 

4 
dictionaries, books on the Korean language, nor Korean 

typewriters available! 

THESTARTOFTHEWAR 

The No1th Koreans launched a massive offensive against South 

Korea on 25 June 1950. This devastating attack was led by 150 

Soviet T-34 tanks, which the South Koreans had no weapon 

powerful enough to stop. In a matter of days, the Korean People's 

Army (ICP A) had captured the South Korean capital, Seoul, and 

was continuing to push south in an apparent attempt to reunify the 

Korean peninsula under communist rule. 

Because of fears of the further spread of Communism and the 

obvious failure of appeasement before World War II, President 

Truman responded quickly. American sea and air power was 

committed to assist South Korea on 27 June, and ground forces 

were committed on 30 June. General Douglas MacArthur, United 

States commander in chief in Asia, transferred units from the 

American army of occupation in Japan to Korea in early July. Due 

to the need to move rapidly to provide assistance to the South 

Koreans, the arriving U.S. troops were out of shape, not well 

equipped, poorly trained, and almost totally unfamiliar with the 

Korean terrain. As a result, the UN forces, consisting mostly of 

Americans and South Koreans, were terribly torn up by the No1th 

Korean attacks. Throughout July, the KPA pushed the UN forces 

south, inflicting dreadful casualties in the process. 

SIGINT PRODUCTION BEGINS 

The lack of attention to the North Korean problem radically 

changed with the 25 June attack. With American troops fighting in 

Korea, the North Korean problem jumped to the top of the priority 

list. Changes were immediately made in the assignments to 

intercept sites. In the two weeks after the sta1t of the war, the 

number of intercept positions collecting North Korean traffic 

increased from two to twelve: two Air Force positions, one Navy, 

and nine Army. The additional coverage was obtained by diverting 

intercept positions from other targets, bringing in additional 

5 
personnel, or having people work overtime. At the start of the war, 

any signal with properties that could be construed as North Korean 

was copied and forwarded to AFSA and ASAPAC for analysis. 

Teletype forwarding to AFSA was introduced to decrease the time 

between intercept and analysis. By 15 October, twenty-three 

positions were dedicated to the North Korean problem. 

In spite of the extremely high interest in North Korean 

communications, the intelligence community realized that 

adequate coverage had to be maintained on the Chinese 

Communist and Soviet targets. Intelligence relating to their 

responses to U.S. activities in Korea and to their support to North 

Korea was vital to American policymakers. Therefore, collection 

resources could not be simply retasked to focus on North Korean 

signals. Reassignments had to be made cautiously to ensure that 

the Chinese and Soviet targets did not suffer too greatly. 

The war also caused immediate analytic changes at AFSA. By 

28 June, data delivery was upgraded so that traffic was arriving at 

AFSA ten to twelve hours after intercept. By 29 June, new traffic 

was arriving hourly in the analytic offices between 0800 and 1630, 

Washington, D.C., time. Analysts now worked on a twenty-fourhour schedule with maximum efforts occurring during the regular 

work hours and an operational watch at night. The additional 

personnel required to perform the analysis on this extended 

schedule were pulled from a variety of places. Analysts working on 

very low priority problems were transferred to the Korean target. 

Other analysts were borrowed from AFSA's technical consultants 

branch. To fill the desperate need for keypunch operators, training 

school students waiting for their clearances were tapped to do the 

punch work, but significant precautions were taken to prevent 

them from finding out the nature and purpose of their work. Any 

linguist with any experience in the Korean language was assigned 

to the problem. Strenuous efforts were made to locate KoreanEnglish dictionaries and to formulate working aids. Further, steps 

were taken to increase the personnel and money at AFSA. 

Once AFSA's analytic efforts were organized, success quickly 

followed. The first plain language translations appeared on 3 July 

1950; the first traffic analysis reports were published on 11 July; 
and the first decrypts were delivered on 14 July. Typewriters that 

could type Korean characters were cobbled together. As of 10 July, 

the traffic analysts had fmther refined their picture of the two 

North Korean traffic networks. One was determined to be a police 

force network of twelve fairly fixed outstations. The other was a 

military network of thirty outstations with sub-nets that was later 

determined to be the principal command net of the North Korean 

Army. Intercepted plaintext translation was well under way by 

mid-July and was hampered only by the lack oflinguists. Even with 

this shortage, the intercepted plaintext was already proving 

extremely helpful to the American effort. Dming the first two 

weeks of July, nine working aids and two Korean language studies 

were completed. Effo1ts were so successful that the 16-31July1950 

issue of the AFSA Semi-Monthly Report for the Office of 

Operations was able to rep01t that 

The Korean traffic is no'' being processed and translations produced 

on a 24-hour basis. This has meant the strengthening of clerical and 

cryptologic aide staff in considerable numbers. The language staff 

has also been bolstered by Joans from other branches. Courses in the 

language are being offered \\ilh a view toward increasing efficiency 

in ... translating. 

By 15 August, the Semi-Monthly Report claimed to have 

brought enough people together to permit full-time operations on 

the Korean Communist problem. By the end of August there were 

fifteen full-time and five part-time personnel engaged in this effort. 

SIGINT AND THE PUSAN PERIMETER 

The KPA continued to force U.S. troops and their allies 

southward until the end of July. Because of his green troops and 

equipment shortages, General Walker decided to establish a 

defensive line, retreat behind it, and hold it at all cost. The Pusan 

Perimeter, as the line came to be called, was an area in the 

southeast corner of South Korea bounded by the Naktong River on 

the west, the Sea of Japan on the east, rugged mountains on the 

north, and the Korea Strait on the south. The perimeter enclosed 

the port city of Pusan, where U.S. troops and supplies were 

7 
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0 
arriving, as well as the city of Taegu, where General Walker set up 

his 8th Army Headquarters. This defensive perimeter was 

established on 31July1950 when the last U.S. soldiers crossed the 

Naktong River. It was held by the U.S. troops for approximately 

forty-five days until the Inchon campaign provided enough of a 

distraction that Walker's forces could push across the Naktong 

River and drive the North Koreans north. 

Transport unloa~s supplies for V.S. hoops in Kore;i <1t PuS<1n. 

Walker stationed his forces in small manned points along the 

perimeter to provide an early warning and some initial defense in 

the case of attack, fading back in the case of a major assault. 

Although the number of troops available to him was growing, 

Walker could not afford to do more at this point. During the first 

two-thirds of August, Walker faced uncoordinated attacks on four 

fronts: at the Naktong Bulge, around Taegu from two directions 

(n01th and east), and in a no1theastern sector around Yongdok, 

Changsa-dong and Pohang-dong. The battle became a "daily 

succession of crises" in which Walker had to rush his men to 

9 
U.S. Army DVKWs bring supplies and equipment to shore From ships at 

Pusan Hatbot, Korea. 

vruious points along the perimeter to stop the North Koreans. 

SIGINT was vital to his efforts at this time because it provided him 

with an indication of the enemy's movements and plans and 

allowed him to spread his men as effectively as possible to repel the 

KPA. 

During this desperate time, the SIGINT repo1ts contained 

interesting and significant information that was very useful in 

efforts to keep the U.S. operations successful. For example, a few of 

the messages detailed locations of several No1th Korean infantry 

battalions. Additional intercept provided a listing of the 

ammunition being delivered to the 1st Division after what they had 

previously held was destroyed by the Americans. Several messages 

discussed a serious radio outage, which prevented an unidentified 

unit from talking to anyone except the N01th Korean 6th Division. 

One message revealed No1th Korean intelligence on the Americans 

and South Koreans. North Korean air force messages desciibed the 

10 
Ma tines pushing to the summit of the Koteqn heights neat the Naktong 

River are taken uncler tire by enemy mortars. 

construction of new runways and some sort of shelters, believed at 

the time to be underground hangars. 

The ground fighting around the Pusan Perimeter was 

temporarily suspended sometime after 21 August. In the ten days 

that followed, the KPA took the time to reorganize, resupply, and 

regroup. This rebuilding was reflected in the messages intercepted 

during this period. The North Koreans ordered massive quantities 

of ammunition from their ordnance department as well as detailed 

maps of the Taegu area in a variety of scales. Follow-up 

communications revealed that most of the ammunition ordered by 

the various North Korean entities was delivered sometime around 

25 August. Another message ordered river-crossing equipment to 

be delivered to the KP A 1st Corps by 23 August. Another described 

the changes of disposition of several regiments and battalions. 

Additionally, North Korean communications described plans to 

organize a rear reconnaissance unit whose mission was to 

annihilate the enemy in fire raids. Obviously the North Koreans 

were preparing for something. 

11 
By late August, General vValker's men were exhausted, and the 

attacks that the North Koreans were planning could have 

potentially pushed them off the Korean peninsula. However, it was 

at about this time that AFSA began processing and delivering the 

messages that would prove the most valuable to the troops inside 

the Pusan Perimeter. These messages contained very specific 

instructions to KP A units, down to the battalion level, of where to 

attack, with what weapons, and when. One message, for example, 

ordered several specific units of North Korean soldiers, including a 

76-millimeter gun battalion, to strike and pursue the enemy and 

attack his flank and rear at several veiy specific points around the 

perimeter. With such intercept, General Walker had a good idea of 

which units would attack and what weapons would be used. Now 

he could plan his strategy accordingly and organize the UN troops 

to best defend the perimeter. 

The North Koreans initiated coordinated assaults on all four 

fronts simultaneously on 31 August. While no surprise, the attacks 

required all of Walker's skills to keep the UN forces intact. The 

North Korean forces broke through the U.S. defenses in Masan, cut 

the U.S. 2nd Division near the Naktong bulge in two, captured 

Pohang-dong, and forced Walker's headquarters to be evacuated 

from Taegu to Pusan. 

Through the first two weeks of September, intercepted 

messages revealed the continuation of a powerful North Korean 

effort. The i5th and ist Divisions, for example, continued to report 

successful advances against Walker's forces. Additional personnel, 

weapons, and ammunition were arriving in forward areas. Steel 

bridges over the North Han, Kum, and South Han Rivers were built 

or restored. However, detailed orders to the North Korean troops 

instructing them where to move and whom to attack continued to 

be sent and were available for General Walker to use in placing his 

troops as effectively as possible. 

In spite of the reported successes, sometime around 5 

September small hints began appearing in communications, 

indicating that there might be some weaknesses in the North 

Korean advance. The arrival of weapons and ammunition at some 

of the North Korean frontline divisions slowed, and Headquarters 
began to instruct officers to make every effort to solve weapons 

problems on their own. A few furious messages appeared 

demanding to know why specific missions were not being carried 

out, such as why tanks weren't advancing and why there were 

complaints about fuel shortages, despite shipments. Successful 

American bombing raids were disrupting transportation. Kwangju 

Airfield was reported as unusable. One unidentified airfield 

demanded that aviation gasoline and other materiel be sent 

immediately. Another reported that air raid shelters were falling 

down, although exactly why this was happening is unclear. 

Nervous messages reported the arrival of additional British and 

American troops in Pusan. Anti-tank mines and entrenching tools 

began arriving in areas around the perimeter. Mine laying and 

sandbagging efforts were reported. Although North Korean 

advances continued through mid-September, indications of 

problems were appearing. 

The final push by UN forces out of the Pusan Perimeter was 

made possible by the UN attack on Inchon, a port city about thirty 

miles west of Seoul. This surprise amphibious assault was the 

brainchild of General MacArthur. He felt that such an attack would 

"catch the enemy by surprise, outflank them, trap them between 

UN forces to the north and to the south, and obviate the need for 

the alternative: a long, bloodier, counteroffensive straight up the 

peninsula." 4 The plan was considered extremely risky by the Joint 

Chiefs and President Truman. These concerns were apparently 

well founded, as an examination of the North Korean 

communications reveals. As early as 6 September, there were 

messages revealing North Korean suspicions of a planned UN 

amphibious assault on a port city on the western coast of South 

Korea. Communications over the next eight days indicated the 

detection of enemy (American) plans to attack Mokp'o, Kunsan, or 

Inchon. There was at least one message requesting verification of a 

rumored American landing at Mokp' o prior to the Inchon attack. 

The 10 September U.S. bombing of Inchon further heightened 

fears, and rapid preparations were made for the defense of Inchon 

in order to prevent enemy landings ashore. 
In spite of these reservations, the Inchon assault was finally 

approved and launched on 15 September. Fortunately for the 

United States, in spite of North Korean suspicions, the attack 

turned out to be a great success. Inchon fell in a day with little loss 

of Amelican life. The American troops in the Pusan Pelimeter, 

aided by the distraction at Inchon, began to push the North 

Koreans to the north at about the same time. UN forces captured 

Seoul by 26 September and had pushed the KP A back across the 

38th parallel by 1 October. The Inchon attack had turned the war 

around. 

Jeeps and ambulances pass two Russian-style tanks knocked out by V.S. 

Marines near the Front in Inchon, Korea. 

Beginning shortly after the American attack at Inchon, North 

Korean communications begin to trace the northern and western 

retreat of the KP A from the Pusan Perimeter zone. Some moved 

back to Kwangju by train, although there was an unspecified 

accident en route that killed three and wounded nineteen. In midSeptember North Korean forces reported from Kumch'on that the 

situation of the battalion was not good and that ammunition was 

needed. There were indications that at least some of the forces in 

Kumch'on were ordered to pull back to Taejon. North Korean 

hospitals were moving their patients to safer areas, and the 

different unit headquarters were shifting north. Warm winter 

clothing could not be delivered to the North Korean troops in the 

mountains in the southern part of the Korean peninsula, and the 

14 
soldiers were getting sick. Obviously the Americans and their allies 

were gaining the upper hand. 

Things quickly fell 

apart for the North 

Koreans after this 

point. The 1-15 

October issue of the 

AFSA Semi-Monthly 

Report of the Office of 

Operations reported 

the disintegration of 

some of the North 

Korean military networks. The 16-31 

October issue reported that AFSA analysis 

was now centered on 

North Korean police 

co mmuni catio n s 

since North Korean 

military traffic had 

vi1tually disappeared. 

The issue further 

A North Korean pHsonet of war captuteq 

by V.S. Marines near Naktong Rivet. 

reported the enemy Supreme Command, now in Sinuiju, and the 

North Koreans were being driven farther and farther north by the 

UN attacks. 

CRITICISMS OF THE AFSA EFFORTS 

In spite of such obvious successes, several individuals 

questioned the praise given AFSA for SIGINT successes during the 

Korean War. For example, in June 1952 General James A. Van 

Fleet, commanding general of the U.S. 8th Army, wrote: "Today, 

our intelligence operations in Korea have not yet approached the 

standards that we reached in the final year of [World War II]." The 

director of NSA concurred with this observation in March 1953. 

The achievements of SIGINT during World War II were far 

superior to those in the Korean War. However, this observation 

15 
must considered in light of the situation in June 1950. At the 

start World War II, analysis of Japanese and German 

communications had been going on for over a decade. By contrast, 

in-depth analysis of North Korean communications did not start 

until after the war began on 25 June 1950. 

Other individuals give credit to the service elements in Korea 

for the SIGINT successes during the war. The problem v.rith this 

view is that until mid-September 1950, ASAPAC and AFSA were 

the only entities available to support the U.S./UN effort. The first 

ASA personnel did not arrive in Korea until 18 September 1950, 

long after the messages so vital to General Walker in the Pusan 

Perimeter campaign had been delivered. The 6oth Signal Service 

Company out of Fmt Lewis, Washington, did not arrive in Korea 

until 9 October 1950 and did not start operations until 16 October. 

There were two groups on the ground in Korea in the early days of 

war: the Air Force Security Service and a South Korean SIGINT 

group working with an American civilian named Nichols. These 

entities, however, did no analysis and concentrated mostly on 

intercepting and translating North Korean communications. While 

their work provided vital tactical intelligence to the UN effort, they 

simply did not have the talent and materials to handle serious longterm projects. AFSA did. 

Still other people argue that the success against N mth Korean 

communications in the early days of the war was due to the poor 

state of North Korean communications security. This is partially 

true. The North Koreans were actually sending highly classified 

materials, like battle plans, in the clear. However, to go from 

almost no effort against North Korean communications to the 

ability to provide significant amounts of valuable intelligence in a 

matter of six weeks was an amazing feat, especially since at the 

beginning of the war there were no full-time North Korean analysts 

or linguists. This situation provides a great testament to the 

abilities of the AFSA analysts who accomplished so much in such 

an incredibly short time. 

But did this success really belong to ASAP AC and not AFSA? In 

studying the individual reports, it is sometimes difficult to 

determine exactly who processed a particular message, AFSA or 
ASAPAC. However, throughout the war, the two organizations 

cooperated very closely, sharing all analytical results. Although, 

because ASAP AC was closer to the action, their reports may have 

arrived in the field before AFSA's, one unit always assisted the 

other. At first, and in fact throughout the time covered by this 

paper, there was a great deal of overlap between the ASAPAC and 

AFSA translation efforts. When the translation times for both 

entities are stated on the reports, they are usually only minutes 

apart. A plan was initiated later in the war to divide the work, 

moving as much of the translation to the field as possible in order 

to speed up turn-around time. Most of the long-term analytic 

effort, however, remained at AFSA throughout the war, and all 

results were passed to ASAP AC and the field elements as quickly as 

possible. 

SIGINT proved vital to the American efforts around the Pusan 

Perimeter. It gave General Walker some warning of the North 

Korean troop movements and allowed him to position his forces 

accordingly. When the North Korean onslaught began to waver, 

SIGINT gave the initial indications of these weaknesses and 

allowed Walker to track their retreat from the Pusan area after the 

UN attack on Inchon. On 24 August 1950, a memo appeared from 

the chief, Office of Operations, detailing the important intelligence 

that AFSA had provided the troops on the ground in Korea. The 

memo stated that AFSA had "confirmed the identification" of 

several North Korean infantry divisions as well as identified several 

previously unknown units; provided "the first evidence of the corps 

structure of the North Korean Army"; provided advance warning of 

enemy troop movements around the Pusan Perimeter; reported on 

the location and construction of airfields and the planes at them; 

located "supply and distribution centers for artillery ammunition"; 

reported evidence that North Korean supplies were running low; 

and provided information on the health and morale of the North 

Korean troops. This priceless support continued through the entire 

campaign and beyond. 

AFSA continued to produce large quantities of meaningful 

intelligence from North Korean traffic until July 1951 when the 
North Koreans introduced new, improved communication security 

practices. However, until that time, AFSA provided the American 

forces in Korea with large quantities of North Korean SIGINT, 

which probably saved them from annihilation. These successes 

clearly demonstrate that AFSA was not ineffective during this 

period, but was a vital part of the war effort. Without the 

contributions of AFSA, the battle at Pusan Perimeter might have 

been lost. 

1. Clay Blair, The Forgotten War (New York: Times Books, 1987), 

2. Ibid. 

3 .. James T. Patterson, Grand Expectations (New York: Oxford 

University Press, 1996), 13-14. 

4. Ibid., 217. 
For further information or additional copies, contact the Center for Cryptologic History, National 

Security Agency, Fort George G. Meade, Maryland 

20755-6886, ATTN: EC 
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