NTPR: Hiroshima and Nagasaki Occupation Forces

Note: For information related to benefit claims, call the Department of Veterans Affairs (VA) at 800-827-1000. 

For information related to participation or dose reconstruction, call the Nuclear Test Personnel Review (NTPR) 

Program at 800-462-3683.

Overview

Atomic bombs were detonated over Hiroshima and Nagasaki, Japan, on August 6 and August 9, 1945, 

respectively. Following the surrender of Japan on August 14, 1945, U.S. forces began occupying the country. 

The first occupation troops arrived in the vicinity of Hiroshima about 60 days after the bombing. The main body 

of occupation troops entered Nagasaki about 45 days after the bombing. In each city, a group of American 

scientists from the Manhattan Engineer District arrived 3 days before these troops and performed a radiological 

survey. However, repatriation of former prisoners of war (POWs) through Nagasaki began before the survey 

and actual occupation of the city.

U.S. troops were in the vicinity of Hiroshima between October 6, 1945, and March 6, 1946, and in the vicinity of 

Nagasaki between September 11, 1945, and July 1, 1946.1

The mission of the occupation was to establish control of the area, ensure compliance with surrender terms, 

and demilitarize the Japanese war machine. The mission did not include the cleanup or any radiological 

decontamination of Hiroshima, Nagasaki, any other areas, or the rebuilding of Japan. 

Units Involved

• Hiroshima - 186th Infantry Regiment of the 41st Division, X Corps of the Sixth Army; later replaced by 

the 34th Infantry Regiment of the 24th Division.

• Nagasaki - 2nd Marine Division, which included the 2nd, 6th, and 8th Regimental Combat 

Teams (RCTs) and an Artillery Group composed principally of the 10th Marine Regiment. Other units of 

the 2nd Marine Division involved were a Headquarters Battalion, Service Troops, an Engineer Group, a 

Tank Battalion, an Observation Squadron, and some smaller organizations. 

Troops were constantly on the move and changing assignments during the occupation, and the duration of 

assignment for any unit in the occupation forces was quite short. Men with the longest service periods were 

given priority for transfer home and whole units were deactivated as it became apparent that large numbers of 

troops were not necessary to fulfill the mission. The size of the occupation force dropped sharply every month.

 1 Some occupation troops remained in the vicinity of Nagasaki later into 1946. However, VA regulations define the 

occupation of Japan as ending on July 1, 1946.
2

The total number of troops occupying Hiroshima was formerly estimated to be about 40,000, and 

approximately 27,000 troops occupied Nagasaki. About 12,000 troops occupied outlying areas within 10 miles 

of either city through July 1, 1946. An additional 118,000 servicemen or more had passed through these areas 

by July 1, 1946. These transient personnel included POWs, troops disembarked for elsewhere in Japan, and 

crews of ships docked nearby. To date, the Nuclear Test Personnel Review Program has identified over 

255,000 veterans who participated in the occupation of Hiroshima and/or Nagasaki.

Refer to Title 38, Code of Federal Regulations, Part 3.309(d)(3) for the complete context of the following 

excerpted formal VA definitions of occupation forces and POWs:

Occupation force: The occupation of Hiroshima or Nagasaki, Japan, by United States forces during the 

period beginning on August 6, 1945, and ending on July 1, 1946.

POW: Internment as a prisoner of war in Japan (or service on active duty in Japan immediately following 

such internment) during World War II, which resulted in an opportunity for exposure to ionizing radiation 

comparable to that of the United States occupation forces in Hiroshima or Nagasaki, Japan, during the 

period beginning on August 6, 1945, and ending on July 1, 1946.

The term “occupation of Hiroshima or Nagasaki, Japan, by United States forces” means official military 

duties within 10 miles of the city limits of either Hiroshima or Nagasaki, Japan, which were required to 

perform or support military occupation functions such as occupation of territory, control of the population, 

stabilization of the government, demilitarization of the Japanese military, rehabilitation of the infrastructure,

or deactivation and conversion of war plants or materials.

Former prisoners-of-war who had an opportunity for exposure to ionizing radiation comparable to that of 

veterans who participated in the occupation of Hiroshima or Nagasaki, Japan, by United States forces shall 

include those who, at any time during the period August 6, 1945, through July 1, 1946:

(A) Were interned within 75 miles of the city limits of Hiroshima or within 150 miles of the city limits of 

Nagasaki, or

(B) Can affirmatively show they worked within the areas set forth in (A) although not interned within 

those areas, or

(C) Served immediately following internment in a capacity which satisfies the above definition of 

occupation forces, or

(D) Were repatriated through the port of Nagasaki. 

Occupation Scenarios

Hiroshima. Elements of the 41st Division landed at Hiro, approximately 10 miles southeast of Hiroshima, on 

October 6, 1945, and secured the Kure Naval Yard. On October 7, the 186th Infantry Regiment of the 

41st Division landed, and the Regiment’s 2nd Battalion established headquarters and billets in Kaidaichi, about 

5 miles southeast of the center of Hiroshima. Since most of the city of Hiroshima had been destroyed by the 

bomb (see Figure 1), no major units were stationed there throughout the occupation. During the next 

2 months, units of the 186th Infantry Regiment conducted reconnaissance patrols and other missions in its 

area of responsibility, including the city of Hiroshima. Records indicate that troops occasionally patrolled the 

destroyed area of the city. Additionally, individuals from nearby units of the 41st could have made brief 

sightseeing trips to view the destruction caused by the bomb. About 900 American POWs were repatriated 

through Hiroshima.

Upon deactivation of the 41st Division in December 1945, the 34th Infantry Regiment of the 24th Division 

took over its mission and moved into the buildings in Kaidaichi originally used by units of the 186th. The 

34th Regiment was responsible for such a wide geographic area that eventually only Company G of the 
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2nd Battalion was stationed in the vicinity of Hiroshima. On March 6, 1946, the 34th Regiment was relieved by 

an Australian Infantry Battalion, and the U.S. occupation in the vicinity of Hiroshima ended.

Nagasaki. Nagasaki was used to repatriate former POWs because the waterfront was sufficiently far from the 

hypocenter (the spot on the ground directly under the detonation, i.e., ground zero) to have escaped most of 

the destructive effects of the bomb and to have been free of radioactivity (see Figure 2). Over 9,000 allied 

POWs (including 2,300 Americans) were processed at Nagasaki from September 11 to 23, 1945. A POW 

recovery team and a detachment of Marine guards were ashore in Nagasaki to support POW processing. 

Additionally, a small advance party of the occupation force (about 12 personnel) arrived in Nagasaki on 

September 16, 1945, and remained until the main force arrived on September 23, 1945.

Upon landing, the 8th RCT and the 10th Marines deployed immediately to Isahaya, about 10 miles north of 

Nagasaki. The 8th RCT did not occupy Nagasaki, but the 10th Marines did so 2 months later. The other 

elements of the 2nd Marine Division debarked in the vicinity of Dejima Wharf and the Mitsubishi shipyard and 

established command posts and billets in those vicinities. The 2nd RCT left Nagasaki in early November, and 

the 6th RCT departed in December 1945 along with two-thirds of the Engineer Group. The Headquarters 

Battalion and portions of the Service Troops left Nagasaki in January 1946. The Tank Battalion, which had 

landed and remained in Fukahori, about 9 miles southeast of Nagasaki, arrived in November 1945 and 

departed the next month. The 10th Marines took over the responsibilities of the 2nd RCT in November, and 

later also those of the 6th RCT. The last units of the 2nd Marine Division left Nagasaki on July 1, 1946.

The specific billet locations of all units have not been precisely determined, but they were undoubtedly outside 

of the radiation survey contours surrounding the hypocenter in Figure 2. An area extending beyond those 

contours was uninhabitable because of complete destruction, and historical documents confirm that the area 

was avoided. As with Hiroshima, presumably patrols and sightseers occasionally entered the areas of residual 

contamination in Nagasaki.

The U.S. Navy transported Marines to Nagasaki and evacuated POWs, but its role ashore was limited. Some 

Navy personnel, including hospital corpsmen, medical and dental officers, chaplains, and a construction 

battalion, were assigned to the 2nd Marine Division.

Radiation Data

Analysis of the scientific data for the Hiroshima and Nagasaki airbursts continues, resulting in revised statistics 

for the detonations.2 The Hiroshima bomb was a uranium-235 weapon that detonated approximately 

1,970 feet above the ground with a yield of 16 kilotons (kt). The Nagasaki bomb was a plutonium-239 weapon 

that detonated at 1,650 feet with a yield of 21 kt. Surveys by the Naval Medical Research Institute (Figures 1 

and 2) show the built-up areas of the respective cities, the hypocenter of each burst, residual gamma radiation 

intensity contours, and the approximate perimeters of total destruction from blast and fire.

The radiological effects of the detonation in each city were similar. Japanese citizens in the vicinity at 

the time of the detonations were exposed to intense radiation produced almost instantaneously. High 

doses of hundreds of rem3 from this initial neutron and gamma radiation contributed to the lethal 

 2 In 2003, the Radiation Effects Research Foundation (RERF) approved a reassessment of the dosimetry system (DS02) 

used to determine prompt radiation doses for atomic-bomb survivors. This study resulted in a slight change of the 

estimated yield and epicenter of the Hiroshima detonation. However, the new information does not affect doses for 

occupation troops, which are based on actual measurements of radiation intensity.

3 A rem is a radiation protection unit of measure that quantifies the risk of biological effects resulting from exposure to 

ionizing radiation. Ionizing radiation is any radiation (gamma, x-ray, beta, neutron, or alpha) capable of displacing 

electrons from atoms or molecules, thereby producing ions. According to the National Council on Radiation Protection 

and Measurements (NCRP, Report No. 160, Table 1.1), the general U.S. population receives about 0.62 rem per year 
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effect on Japanese citizens located beneath the bursts. This initial radiation only occurs for about 

one minute after a nuclear detonation and does not persist thereafter.

In contrast, the earliest residual radiation levels encountered by Japanese were survivable. Both burst 

altitudes were sufficiently high that bomb debris did not reach the ground in the vicinity of the hypocenter. After 

the detonations, strong thermal updrafts were produced which lifted the radioactive bomb debris and smoke 

into “mushroom” clouds. Most of the mixed debris settled to the ground as radioactive fallout downwind of the 

cities. In each city, there was one area of low-level residual radioactivity in a roughly circular area caused by 

neutron activation of soil and building materials around the hypocenter. Additionally, there was a second area 

of residual radioactivity located downwind and outside the city, caused by fallout carried to the ground during 

rain shower activity within an hour after the detonation. Subsequent heavy rainfall washed away some of the 

residual radioactivity. During the intervening weeks before the occupation forces arrived, this rainfall, 

combined with radiological decay, reduced the radiation levels from fallout and neutron-activated materials by 

a factor of several thousand. This explains, in part, why the radiation doses of occupation forces were at least 

a thousand times lower than those of Japanese citizens located near ground zero at the time of the detonation.

Based on radiation surveys performed by American scientists from the Manhattan Engineer District prior to the 

arrival of the occupation forces, the greatly-decayed residual radioactivity levels in and around Hiroshima and 

Nagasaki at the time the occupation forces arrived were such that military activities could proceed as planned, 

unimpeded by radiological considerations.

The Nishiyama Reservoir had the highest radiation measurement recorded at the time of the troops’ arrival. 

However, this area was remote and rugged, with steep slopes and heavy forests, few trails or roads, and even 

fewer buildings. The Japanese population in the area was sparse, so there were no occupation forces 

stationed in the vicinity, and little need for military patrols into the area.

Personnel doses

Dose reconstructions are based on (1) residual radiation measurements, documented and published shortly 

after the bombings, (2) extensive review and analysis of the residual radioactivity in ensuing decades, 

(3) veteran activities, (4) the documented arrival and departure dates of each military unit which operated in the 

vicinity of Hiroshima and Nagasaki, (5) and potential fallout contact exposure.

Doses received by veterans at Nagasaki were generally low; the approximate maximum external dose based 

on a presence for entire interval from September 16, 1945, to July 1, 1946, is 1.25 rem. Potential exposure to

occupation troops assigned to Hiroshima was markedly lower due to radioactive decay prior to the delayed

October 6, 1945, entry into the city. External doses received by Kumamoto POWs for the interval from 

detonation on August 9, 1945, to a repatriation date of approximately September 23, 1945, is < 1 rem. Skin 

dose estimates for these POWs assume lack of sheltering and continuous contact with fallout contaminated 

surfaces since the POWs slept on the ground.

Doses to Japanese Survivors

The doses noted in the previous paragraph are orders of magnitude lower than the reconstructed initial 

radiation doses for the hundreds of thousands of Japanese survivors, which ranged up to hundreds of rem. 

Their health continues to be monitored by the Radiation Effects Research Foundation (http://www.rerf.or.jp/). 

For further information on RERF or the studies of Japanese atomic bomb survivors, contact:

 

from natural background radiation sources (radon, cosmic rays, and rocks) and man-made radiation sources (medical 

diagnostic x-rays and consumer products).
5

Department of Energy Office of Health and Safety 

Office of Domestic and International Health Studies

Forrestal Building

1000 Independence Avenue, SW

Washington, DC 20585

https://www.energy.gov/ehss/services/worker-health-and-safety/domestic-health-studies-and-activities

For more information, see the report “Radiation Dose Reconstruction, U.S. Occupation Forces in Hiroshima 

and Nagasaki, Japan, 1945-1946” (DNA 5512F), available online at:

http://www.dtra.mil/DTRA-Mission/Reference-Documents/NTPR-info/.

September 2021
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Figure 1. Results of the Naval Medical Research Institute (NMRI) survey performed in Hiroshima on 

November 1-2, 1945, showing residual radiation levels of 0.069 milliroentgen per hour (mR/hr) in the vicinity of 

ground zero4 and 0.011 mR/hr at the outermost contour. Source: DNA 5512F.

 4 The NMRI survey report (Measurement of the Residual Radiation Intensity at the Hiroshima and Nagasaki Atomic Bomb 

Sites, NMRI-160A) documents a residual exposure rate of 0.081 mR/hr at the hypocenter, as well as spot measurements 

of 0 and 9 mR/hr among the “scattered points” to the west of the city center. These values are not represented on the 

map in DNA 5512F.
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Figure 2. Results of the NMRI survey performed in Nagasaki on October 15-27, 1945, showing residual 

radiation levels of 0.072 mR/hr (maximum) at ground zero and up to 1.08 mR/hr in the rainout area at the 

Nishiyama Reservoir. Scientific analysis of these data indicated that two radionuclides resulting from neutron 

activation of surface soil and building materials (scandium-46 and cobalt-60) produced the radiation levels near 

ground zero in both Hiroshima and Nagasaki. Fission product radionuclides produced the radiation levels in 

the rainout areas. Source: DNA 5512F.