Unit 731 (Japanese: 731, Hepburn: Nana-san-ichi Butai) was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army that undertook lethal human experimentation during the Second Sino-Japanese War (19371945) of World War II. It was responsible for some of the most notorious war crimes carried out by Imperial Japan. Unit 731 was based at the Pingfang district of Harbin, the largest city in the Japanese puppet state of Manchukuo (now Northeast China).

It was officially known as the Epidemic Prevention and Water Purification Department of the Kwantung Army (, Kantgun Beki Kysuibu Honbu). Originally set up under the Kempeitai military police of the Empire of Japan, Unit 731 was taken over and commanded until the end of the war by General Shir Ishii, a combat medic officer in the Kwantung Army. The facility itself was built between 1934 and 1939 and officially adopted the name "Unit 731" in 1941.

At least 3,000 men, women, and children[1][2]from which at least 600 every year were provided by the Kempeitai[3] were subjected as "logs" to experimentation conducted by Unit 731 at the camp based in Pingfang alone, which does not include victims from other medical experimentation sites, such as Unit 100.[4]

Unit 731 participants of Japan attest that most of the victims they experimented on were Chinese while a lesser percentage were Soviet, Mongolian, Korean, and other Allied POWs. The unit received generous support from the Japanese government up to the end of the war in 1945.

Instead of being tried for war crimes after the war, the researchers involved in Unit 731 were secretly given immunity by the U.S. in exchange for the data they gathered through human experimentation.[5] Other researchers that the Soviet forces managed to arrest first were tried at the Khabarovsk War Crime Trials in 1949. The Americans did not try the researchers so that the information and experience gained in bio-weapons could be co-opted into the U.S. biological warfare program, much as they had done with German researchers in Operation Paperclip.[6] On 6 May 1947, Douglas MacArthur, as Supreme Commander of the Allied Forces, wrote to Washington that "additional data, possibly some statements from Ishii, can probably be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as 'War Crimes' evidence".[5] Victim accounts were then largely ignored or dismissed in the West as communist propaganda.[7]

In 1932, Surgeon General Shir Ishii ( Ishii Shir), chief medical officer of the Japanese Army and protg of Army Minister Sadao Araki was placed in a command of the Army Epidemic Prevention Research Laboratory (AEPRL). Ishii organized a secret research group, the "Tg Unit", for various chemical and biological experimentation in Manchuria. Ishii had proposed the creation of a Japanese biological and chemical research unit in 1930, after a two-year study trip abroad, on the grounds that Western powers were developing their own programs. One of Ishii's main supporters inside the army was Colonel Chikahiko Koizumi, who later became Japan's Health Minister from 1941 to 1945. Koizumi had joined a secret poison gas research committee in 1915, during World War I, when he and other Imperial Japanese Army officers became impressed by the successful German use of chlorine gas at the Second Battle of Ypres, where the Allies suffered 5,000 deaths and 15,000 wounded as a result of the chemical attack.[8][9]

Unit Tg was implemented in the Zhongma Fortress, a prison/experimentation camp in Beiyinhe, a village 100km (62mi) south of Harbin on the South Manchuria Railway. A jailbreak in autumn 1934 and later explosion (believed to be an attack) in 1935 led Ishii to shut down Zhongma Fortress. He received the authorization to move to Pingfang, approximately 24km (15mi) south of Harbin, to set up a new and much larger facility.[10]

In 1936, Emperor Hirohito authorized by decree the expansion of this unit and its integration into the Kwantung Army as the Epidemic Prevention Department.[11] It was divided at the same time into the "Ishii Unit" and "Wakamatsu Unit" with a base in Hsinking. From August 1940, the units were known collectively as the "Epidemic Prevention and Water Purification Department of the Kwantung Army ()"[12] or "Unit 731" (731) for short. In addition to the establishment of Unit 731, the decree also called for the establishment of an additional biological warfare development unit called the Kwantung Army Military Horse Epidemic Prevention Workshop (later referred to as Manchuria Unit 100) and a chemical warfare development unit called the Kwantung Army Technical Testing Department. (later referred to as Manchuria Unit 516) After the Japanese invasion of China in 1937, sister chemical and biological warfare units were founded in major Chinese cities, and were referred to as Epidemic Prevention and Water Supply Units. Detachments included Unit 1855 in Beijing, Unit Ei 1644 in Nanjing, Unit 8604 in Guangzhou and later, Unit 9420 in Singapore. The compilation of all these units comprised Ishiis network, and at its height in 1939, was composed of more than 10,000 personnel.[13]

Medical doctors and professors from Japan were attracted to join Unit 731 by the rare opportunity to conduct human experimentation and strong financial support from the Army.[14]

A special project code-named Maruta used human beings for experiments. Test subjects were gathered from the surrounding population and were sometimes referred to euphemistically as "logs" (, maruta), used in such contexts as "How many logs fell?". This term originated as a joke on the part of the staff because the official cover story for the facility given to the local authorities was that it was a lumber mill. However, in an account by a man who worked as a junior uniformed civilian employee of the Imperial Japanese Army in Unit 731, the project was internally called "Holzklotz", which is a German word for log.[15] In a further parallel, the corpses of "sacrificed" subjects were disposed of by incineration.[16] Researchers in Unit 731 also published some of their results in peer-reviewed journals, writing as though the research had been conducted on non-human primates called "Manchurian monkeys" or "long-tailed monkeys".[17]

The test subjects were selected to give a wide cross-section of the population and included common criminals, captured bandits and anti-Japanese partisans, political prisoners and also people rounded up by the Kempeitai military police for alleged "suspicious activities". They included infants, the elderly, and pregnant women. The members of the unit, approximately three hundred researchers, included doctors and bacteriologists; most were Japanese, although some were Chinese and Korean collaborators.[18] Many had been desensitized to performing unpleasant experiments from experience in animal research.[19]

Thousands of men, women, children, and infants interned at prisoner of war camps were subjected to vivisection, often without anesthesia and usually ending with the death of the victim.[20][21] Vivisections were performed on prisoners after infecting them with various diseases. Researchers performed invasive surgery on prisoners, removing organs to study the effects of disease on the human body. These were conducted while the patients were alive because it was thought that the death of the subject would affect the results.[22]

Prisoners had limbs amputated in order to study blood loss. Those limbs that were removed were sometimes re-attached to the opposite sides of the body. Some prisoners had their stomachs surgically removed and the esophagus reattached to the intestines. Parts of organs, such as the brain, lungs, and liver, were removed from some prisoners.[21] Imperial Japanese Army surgeon Ken Yuasa suggests that the practice of vivisection on human subjects (mostly Chinese communists or common criminals) was widespread even outside Unit 731,[23] estimating that at least 1,000 Japanese personnel were involved in the practice in mainland China.[24]

Prisoners were injected with diseases, disguised as vaccinations,[25] to study their effects. To study the effects of untreated venereal diseases, male and female prisoners were deliberately infected with syphilis and gonorrhoea, then studied. Prisoners were also repeatedly subject to rape by guards.[26]

Plague fleas, infected clothing and infected supplies encased in bombs were dropped on various targets. The resulting cholera, anthrax, and plague were estimated to have killed at least 400,000 Chinese civilians.[27] Tularemia was tested on Chinese civilians.[28]

Unit 731 and its affiliated units (Unit 1644 and Unit 100 among others) were involved in research, development and experimental deployment of epidemic-creating biowarfare weapons in assaults against the Chinese populace (both civilian and military) throughout World War II. Plague-infected fleas, bred in the laboratories of Unit 731 and Unit 1644, were spread by low-flying airplanes upon Chinese cities, including coastal Ningbo in 1940, and Changde, Hunan Province, in 1941. This military aerial spraying killed tens of thousands of people with bubonic plague epidemics.[29][30][31]

It is possible that Unit 731's methods and objectives were also followed in Indonesia, in a case of a failed experiment designed to validate a synthesized tetanus toxoid vaccine.[32]

Physiologist Yoshimura Hisato conducted experiments by taking captives outside, dipping various appendages into water, and allowing the limb to freeze. Once frozen, which testimony from a Japanese officer said "was determined after the 'frozen arms, when struck with a short stick, emitted a sound resembling that which a board gives when it is struck'",[33] ice was chipped away and the area doused in water. The effects of different water temperatures were tested by bludgeoning the victim to determine if any areas were still frozen.

Doctors orchestrated forced sex acts between infected and non-infected prisoners to transmit the disease, as the testimony of a prison guard on the subject of devising a method for transmission of syphilis between patients shows:

"Infection of venereal disease by injection was abandoned, and the researchers started forcing the prisoners into sexual acts with each other. Four or five unit members, dressed in white laboratory clothing completely covering the body with only eyes and mouth visible, handled the tests. A male and female, one infected with syphilis, would be brought together in a cell and forced into sex with each other. It was made clear that anyone resisting would be shot."[34]

After victims were infected, they were vivisected at different stages of infection, so that internal and external organs could be observed as the disease progressed. Testimony from multiple guards blames the female victims as being hosts of the diseases, even as they were forcibly infected. Genitals of female prisoners that were infected with syphilis were called "jam filled buns" by guards.[35]

Some children grew up inside the walls of Unit 731, infected with syphilis. A Youth Corps member deployed to train at Unit 731 recalled viewing a batch of subjects that would undergo syphilis testing: "one was a Chinese woman holding an infant, one was a White Russian woman with a daughter of four or five years of age, and the last was a White Russian woman with a boy of about six or seven."[35] The children of these women were tested in ways similar to their parents, with specific emphasis on determining how longer infection periods affected the effectiveness of treatments.

Female prisoners were forced to become pregnant for use in experiments. The hypothetical possibility of vertical transmission (from mother to child) of diseases, particularly syphilis, was the stated reason for the torture. Fetal survival and damage to mother's reproductive organs were objects of interest. Though "a large number of babies were born in captivity", there have been no accounts of any survivors of Unit 731, children included. It is suspected that the children of female prisoners were killed after birth or aborted.[35]

While male prisoners were often used in single studies, so that the results of the experimentation on them would not be clouded by other variables, women were sometimes used in bacteriological or physiological experiments, sex experiments, and as the victims of sex crimes. The testimony of a unit member that served as guard graphically demonstrated this reality:

"One of the former researchers I located told me that one day he had a human experiment scheduled, but there was still time to kill. So he and another unit member took the keys to the cells and opened one that housed a Chinese woman. One of the unit members raped her; the other member took the keys and opened another cell. There was a Chinese woman in there who had been used in a frostbite experiment. She had several fingers missing and her bones were black, with gangrene set in. He was about to rape her anyway, then he saw that her sex organ was festering, with pus oozing to the surface. He gave up the idea, left and locked the door, then later went on to his experimental work."[35]

Human targets were used to test grenades positioned at various distances and in different positions. Flamethrowers were tested on humans. Humans were also tied to stakes and used as targets to test pathogen-releasing bombs, chemical weapons, and explosive bombs.[36][37]

In other tests, subjects were deprived of food and water to determine the length of time until death; placed into high-pressure chambers until death; experimented upon to determine the relationship between temperature, burns, and human survival; placed into centrifuges and spun until death; injected with animal blood; exposed to lethal doses of x-rays; subjected to various chemical weapons inside gas chambers; injected with sea water; and burned or buried alive.[38]

Some tests had no medical purpose at all with instead intent to administer excruciating pain, such as injecting horse urine into prisoners' kidneys.[27]

Japanese researchers performed tests on prisoners with bubonic plague, cholera, smallpox, botulism, and other diseases.[39] This research led to the development of the defoliation bacilli bomb and the flea bomb used to spread bubonic plague.[40] Some of these bombs were designed with porcelain shells, an idea proposed by Ishii in 1938.

These bombs enabled Japanese soldiers to launch biological attacks, infecting agriculture, reservoirs, wells, and other areas with anthrax, plague-carrier fleas, typhoid, dysentery, cholera, and other deadly pathogens. During biological bomb experiments, researchers dressed in protective suits would examine the dying victims. Infected food supplies and clothing were dropped by airplane into areas of China not occupied by Japanese forces. In addition, poisoned food and candies were given to unsuspecting victims, and the results examined.[citation needed]

In 2002, Changde, China, site of the plague flea bombing, held an "International Symposium on the Crimes of Bacteriological Warfare" which estimated that the number of people killed by the Imperial Japanese Army germ warfare and human experiments was around 580,000.[30] The American historian Sheldon H. Harris claims that 200,000 died.[31] In addition to Chinese casualties, 1,700 Japanese in Chekiang were killed by their own biological weapons while attempting to unleash the biological agent, indicating serious issues with distribution.[1]

During the final months of World War II, Japan planned to use plague as a biological weapon against San Diego, California. The plan was scheduled to launch on September 22, 1945, but Japan surrendered five weeks earlier.[41][42][43][44]

According to A.S. Wells, the majority of victims were mostly Chinese (including accused "bandits" and "Communists"), Korean, and Soviet, although they may also have included European, American, Indian, and Australian prisoners of war.[45]

Unit 731 participants of Japan attest that most of the victims they experimented on were Chinese[23] while a small percentage were Soviet, Mongolian, Korean, and other Allied POWs.[46] Almost 70% of the victims who died in the Pingfang camp were Chinese, including both civilian and military.[47] Close to 30% of the victims were Soviet.[48] Some others were Southeast Asians and Pacific Islanders, at the time colonies of the Empire of Japan, and a small number of Allied prisoners of war.[49]

Robert Peaty (19031989), a British Major in the Royal Army Ordnance Corps, was the senior ranking allied officer. During this time, he kept a secret diary. A copy of his entire diary exists in the NARA archives.[50] An extract of the diary is available at the UK National Archives at Kew.[51] He was interviewed by the Imperial War Museum in 1981, and the audio recording tape reels are in the IWM's archives.[52]

In April 2018, the National Archives of Japan for the first time disclosed a nearly complete list of 3607 people who worked for Unit 731 to Dr. Katsuo Nishiyama of the Shiga University of Medical Science, who says that he intends to publish the list online.[53]

Unit 731 was divided into eight divisions:

The Unit 731 complex covered six square kilometres (2.3 square miles) and consisted of more than 150 buildings. The design of the facilities made them hard to destroy by bombing. The complex contained various factories. It had around 4,500 containers to be used to raise fleas, six cauldrons to produce various chemicals, and around 1,800 containers to produce biological agents. Approximately 30 kilograms (66 pounds) of bubonic plague bacteria could be produced in a few days.

Some of Unit 731's satellite facilities are in use by various Chinese industrial concerns. A portion has been preserved and is open to visitors as a War Crimes Museum.

A medical school and research facility belonging to Unit 731 operated in the Shinjuku District of Tokyo during World War II. In 2006, Toyo Ishiia nurse who worked at the school during the warrevealed that she had helped bury bodies and pieces of bodies on the school's grounds shortly after Japan's surrender in 1945. In response, in February 2011 the Ministry of Health began to excavate the site.[55]

China requested DNA samples from any human remains discovered at the site. The Japanese governmentwhich has never officially acknowledged the atrocities committed by Unit 731rejected the request.[56]

The related Unit 8604 was operated by the Japanese Southern China Area Army and stationed at Guangzhou (Canton). This installation conducted human experimentation in food and water deprivation as well as water-borne typhus. According to postwar testimony, this facility served as the main rat breeding farm for the medical units to provide them with bubonic plague vectors for experiments.[57]

Unit 731 was part of the Epidemic Prevention and Water Purification Department which dealt with contagious disease and water supply generally.

Operations and experiments continued until the end of the war. Ishii had wanted to use biological weapons in the Pacific War since May 1944, but his attempts were repeatedly snubbed.

With the coming of the Red Army in August 1945, the unit had to abandon their work in haste. The members and their families fled to Japan.

Ishii ordered every member of the group "to take the secret to the grave", threatening to find them if they failed, and prohibiting any of them from going into public work back in Japan. Potassium cyanide vials were issued for use in the event that the remaining personnel were captured.

Skeleton crews of Ishii's Japanese troops blew up the compound in the final days of the war to destroy evidence of their activities, but most were so well constructed that they survived somewhat intact.

Among the individuals in Japan after its 1945 surrender was Lieutenant Colonel Murray Sanders, who arrived in Yokohama via the American ship Sturgess in September 1945. Sanders was a highly regarded microbiologist and a member of America's military center for biological weapons. Sanders' duty was to investigate Japanese biological warfare activity. At the time of his arrival in Japan he had no knowledge of what Unit 731 was.[35] Until Sanders finally threatened the Japanese with bringing the Soviets into the picture, little information about biological warfare was being shared with the Americans. The Japanese wanted to avoid prosecution under the Soviet legal system, so the next morning after he made his threat, Sanders received a manuscript describing Japan's involvement in biological warfare.[58] Sanders took this information to General Douglas MacArthur, who was the Supreme Commander of the Allied Powers responsible for rebuilding Japan during the Allied occupations. MacArthur struck a deal with Japanese informants:[59] he secretly granted immunity to the physicians of Unit 731, including their leader, in exchange for providing America, but not the other wartime allies, with their research on biological warfare and data from human experimentation.[5] American occupation authorities monitored the activities of former unit members, including reading and censoring their mail.[60] The U.S. believed that the research data was valuable, and did not want other nations, particularly the Soviet Union, to acquire data on biological weapons.[61]

The Tokyo War Crimes Tribunal heard only one reference to Japanese experiments with "poisonous serums" on Chinese civilians. This took place in August 1946 and was instigated by David Sutton, assistant to the Chinese prosecutor. The Japanese defense counsel argued that the claim was vague and uncorroborated and it was dismissed by the tribunal president, Sir William Webb, for lack of evidence. The subject was not pursued further by Sutton, who was probably unaware of Unit 731's activities. His reference to it at the trial is believed to have been accidental.

Although publicly silent on the issue at the Tokyo Trials, the Soviet Union pursued the case and prosecuted twelve top military leaders and scientists from Unit 731 and its affiliated biological-war prisons Unit 1644 in Nanjing, and Unit 100 in Changchun, in the Khabarovsk War Crime Trials. Included among those prosecuted for war crimes, including germ warfare, was General Otoz Yamada, the commander-in-chief of the million-man Kwantung Army occupying Manchuria.

The trial of those captured Japanese perpetrators was held in Khabarovsk in December 1949. A lengthy partial transcript of the trial proceedings was published in different languages the following year by a Moscow foreign languages press, including an English language edition.[62] The lead prosecuting attorney at the Khabarovsk trial was Lev Smirnov, who had been one of the top Soviet prosecutors at the Nuremberg Trials. The Japanese doctors and army commanders who had perpetrated the Unit 731 experiments received sentences from the Khabarovsk court ranging from two to 25 years in a Siberian labor camp. The U.S. refused to acknowledge the trials, branding them communist propaganda.[63] The sentences doled out to the Japanese perpetrators were unusually lenient for Soviet standards, and all but one of the defendants returned to Japan by the 1950s (with the remaining prisoner committing suicide inside his cell). In addition to the accusations of propaganda, the US also asserted that the trials were to only serve as a distraction from the Soviet treatment of several hundred thousand Japanese prisoners of war; meanwhile, the USSR asserted that the US had given the Japanese diplomatic leniency in exchange for information regarding their human experimentation. The accusations of both the US and the USSR were true, and it is believed that they had also given information to the Soviets regarding their biological experimentation for judicial leniency.[64] This was evidenced by the Soviet Union building a biological weapons facility in Sverdlovsk using documentation captured from Unit 731 in Manchuria.[65]

As above, under the American occupation the members of Unit 731 and other experimental units were allowed to go free. One graduate of Unit 1644, Masami Kitaoka, continued to do experiments on unwilling Japanese subjects from 1947 to 1956 while working for Japan's National Institute of Health Sciences. He infected prisoners with rickettsia and mental health patients with typhus.[66]

Shiro Ishii, as the chief of the unit, was granted war crime immunity from the US occupation authorities, because of his provision of human experimentation research materials to the US. From 1948 to 1958, less than 5% of the documents were transferred onto microfilm and stored in the National Archives of the United States, before being shipped back to Japan.[67]

Japanese discussions of Unit 731's activity began in the 1950s, after the end of the American occupation of Japan. In 1952, human experiments carried out in Nagoya City Pediatric Hospital, which resulted in one death, were publicly tied to former members of Unit 731.[68] Later in that decade, journalists suspected that the murders attributed by the government to Sadamichi Hirasawa were actually carried out by members of Unit 731. In 1958, Japanese author Shsaku End published the book The Sea and Poison about human experimentation, which is thought to have been based on a real incident.

The author Seiichi Morimura published The Devil's Gluttony () in 1981, followed by The Devil's Gluttony: A Sequel in 1983. These books purported to reveal the "true" operations of Unit 731, but actually confused them with that of Unit 100, and falsely used unrelated photos attributing them to Unit 731, which raised questions about their accuracy.[69][70]

Also in 1981 appeared the first direct testimony of human vivisection in China, by Ken Yuasa. Since then many more in-depth testimonies have appeared in Japanese. The 2001 documentary Japanese Devils was composed largely of interviews with 14 members of Unit 731 who had been taken as prisoners by China and later released.[71]

Since the end of the Allied occupation, the Japanese government has repeatedly apologized for its pre-war behavior in general, but specific apologies and indemnities are determined on the basis of bilateral determination that crimes occurred, which requires a high standard of evidence. Unit 731 presents a special problem, since unlike Nazi human experimentation which the U.S. publicly condemned, the activities of Unit 731 are known to the general public only from the testimonies of willing former unit members, and testimony cannot be employed to determine indemnity in this way.

Japanese history textbooks usually contain references to Unit 731, but do not go into detail about allegations, in accordance with this principle.[72][73] Sabur Ienaga's New History of Japan included a detailed description, based on officers' testimony. The Ministry for Education attempted to remove this passage from his textbook before it was taught in public schools, on the basis that the testimony was insufficient. The Supreme Court of Japan ruled in 1997 that the testimony was indeed sufficient and that requiring it to be removed was an illegal violation of freedom of speech.[74]

In 1997, the international lawyer Knen Tsuchiya filed a class action suit against the Japanese government, demanding reparations for the actions of Unit 731, using evidence filed by Professor Makoto Ueda of Rikkyo University. All Japanese court levels found that the suit was baseless. No findings of fact were made about the existence of human experimentation, but the decision of the court was that reparations are determined by international treaties and not by national court cases.[citation needed]

In August 2002, the Tokyo district court ruled for the first time that Japan had engaged in biological warfare. Presiding judge Koji Iwata ruled that the Unit 731, on the orders of the Imperial Japanese Army headquarters, used bacteriological weapons on Chinese civilians between 1940 and 1942, spreading diseases including plague and typhoid in the cities of Quzhou, Ningbo and Changde. However, he rejected the victims' claims for compensation on the grounds that they had already been settled by international peace treaties.[75]

In October 2003, a member of the House of Representatives of Japan filed an inquiry. Japanese Prime Minister Junichiro Koizumi responded that the Japanese government did not then possess any records related to Unit 731, but the government recognized the gravity of the matter and would publicize any records that were located in the future.[76] In April 2018, the National Archives of Japan released the names of 3,607 members of Unit 731, in response to a request by Professor Katsuo Nishiyama of the Shiga University of Medical Science.[77][78]

After WWII, the Office of Special Investigations created a watchlist of suspected Axis collaborators and persecutors that are banned from entering the U.S. While they have added over 60,000 names to the watchlist, they have only been able to identify under 100 Japanese participants. In a 1998 correspondence letter between the DOJ and Rabbi Abraham Cooper, Eli Rosenbaum, director of OSI, stated that this was due to two factors. (1) While most documents captured by the U.S. in Europe were microfilmed before being returned to their respective governments, the Department of Defense decided to not microfilm its vast collection of documents before returning them back to the Japanese government. (2) The Japanese government has also failed to grant the OSI meaningful access to these and related records after the war, while European countries, on the other hand, have been largely cooperative.[79] The cumulative effect of which is that information pertaining to identifying these individuals is, in effect, impossible to recover.

There have been several films about the atrocities of Unit 731.

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Unit 731 - Wikipedia