Unit 731 (731部隊, Nana-san-ichi Butai) was a secret biological and chemical warfare research facility run by the Imperial Japanese Army between 1933 and 1945. Officially known as the Manchu Detachment 731, it was also referred to as the Kamo Detachment[6] and the Ishii Unit.

The main facility was based in the Pingfang district of Harbin, located within the Japanese puppet state of Manchukuo (which is now Northeast China). From there, the unit managed multiple branches across mainland China and Southeast Asia under the "Epidemic Prevention Department" umbrella. During the Second Sino-Japanese War, the unit launched the largest biological weapons attacks in history. They killed an estimated 200,000 people in various cities and villages by intentionally poisoning water supplies, food, and crops.[7]

The facility was led by General Shirō Ishii and received strong support from the Japanese military. An estimated 14,000 people were killed by human experimentation in the facility itself.[8] Its activities included infecting prisoners with deadly diseases, conducting vivisection, performing organ harvesting, testing hypobaric chambers, amputating limbs, and exposing victims to chemical agents and explosives. Prisoners—often referred to as "logs" (or "maruta") by the staff—were mainly Chinese civilians, but also included Russians, Koreans, and others, including children and pregnant women. No documented survivors are currently known.

After the war, twelve Unit 731 members were tried by the USSR in the 1949 Khabarovsk war crimes trials and sentenced to prison. However, many key figures, including Ishii, were granted immunity in a coverup by the United States in exchange for their research data. The Truman administration concealed the unit's crimes and paid stipends to former personnel.[9][10] The obtained information was not of significant value, as the US biological weapons program had surpassed the capabilities of Unit 731.[11] The USSR handed out relatively lenient sentences at the Khabarovsk war crimes trials.[12][13]

On 28 August 2002, the Tokyo District Court acknowledged that Japan had conducted biological warfare in China and held the state responsible for related deaths, but rejected compensation, arguing this was provided by postwar treaties such as the Treaty of San Francisco. In April 2018, the National Archives of Japan released a near-complete list of 3,607 personnel affiliated with Unit 731. The Japanese government has never issued an official acknowledgement of its biological warfare.[14][15]

Formations

Building of the Unit 731 bioweapon facility in Harbin

The Empire of Japan initiated its biological weapons program during the 1930s, due to the prohibition of biological weapons in interstate conflicts by the Geneva Protocol of 1925. They reasoned that the ban implied their efficacy as weapons.[1] Japan's occupation of Manchuria began in 1931, after the Japanese invasion of Manchuria.[16] Japan decided to build Unit 731 in Manchuria because the occupation not only gave the Japanese an advantage of separating the research station from their island but also gave them access to as many Chinese individuals as they wanted for use as test subjects.[16] They viewed the Chinese as no-cost assets and hoped this ready supply of test subjects would give them a competitive advantage in biological warfare.[16] Most of the victims were Chinese, but many victims were also of different nationalities.[1] These facilities contained more than just medical research and experimentation areas; they also included spaces for detaining victims, essentially functioning as a prison.[17] The research and experimentation rooms were constructed around the detention area, allowing researchers to conduct their daily work while monitoring the prisoners.[17] Founded in 1933, Unit 731 expanded to include 3,000 staff members, 150 structures, and the capacity to detain up to 600 prisoners concurrently for experimental purposes.[18]

Unit 731 was a clandestine division of the Japanese Kwantung Army based in Manchuria during World War II. Led by Lieutenant General Shirō Ishii, the organization dedicated to the advancement of biological weaponry within the imperial army was commonly referred to as the Ishii Network.[19] The Ishii Network was headquartered at the Epidemic Prevention Research Laboratory, established in 1932 at the Japanese Army Military Medical School in Tokyo. Unit 731 was the first of several covert units established as offshoots of the research lab, serving as field stations and experimental sites to advance biological warfare techniques. These efforts culminated in the experimental deployment of biological weapons on Chinese cities, a direct breach of the 1925 Geneva Protocol prohibiting the use of biological and chemical weapons in warfare. Participants in these activities were aware of the violations and recognized the inhumanity of using human subjects in laboratory experiments, prompting the establishment of Unit 731 and other secret units.[19]

Under the direction of Shirō Ishii, the Epidemic Prevention Research Laboratory was established following his return from a two-year exploration of American and European research institutions. With the endorsement of high-ranking military officials, it was established for the purpose of developing biological weapons. Ishii aimed to create biological weapons with humans as their intended victims, and Unit 731 was formed specifically to pursue this objective.[19] Ishii organized a secret research group, the "Tōgō Unit", for chemical and biological experimentation in Manchuria.[19]

In 1936, Emperor Hirohito issued a decree authorizing the expansion of the unit and its integration into the Kwantung Army as the Epidemic Prevention Department.[20] It was divided at that time into the "Ishii Unit" and the "Wakamatsu Unit", with a base in Xinjing. From August 1940 on, the units were known collectively as the "Epidemic Prevention and Water Purification Department of the Kwantung Army" or "Unit 731" for short.[21]

One of Ishii's main supporters inside the army was Colonel Chikahiko Koizumi, who later served as 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 were impressed by the successful German use of chlorine gas at the Second Battle of Ypres, in which The Entente suffered 6,000 deaths and 15,000 wounded as a result of the chemical attack.[22]: 8–9 [23]

Zhongma Fortress

Unit Tōgō was set into motion in the Zhongma Fortress, a prison and experimentation camp in Beiyinhe, a village 100 kilometers (62 mi) south of Harbin on the South Manchuria Railway. The prisoners brought to Zhongma included common criminals, captured bandits, and anti-Japanese partisans, as well as political prisoners and people rounded up on false charges by the Kempeitai. Prisoners were generally well fed on a diet of rice or wheat, meat, fish, and occasionally even alcohol, in order to be in normal health at the beginning of experiments. Then, over several days, prisoners were eventually drained of blood and deprived of nutrients and water. Their deteriorating health was recorded. Some were also vivisected. Others were deliberately infected with plague bacteria and other microbes. A prison break in the autumn of 1934, which jeopardized the facility's secrecy, and an explosion in 1935, which was believed to be sabotage, led Ishii to shut down the Zhongma Fortress. He then received authorization to relocate to Pingfang, approximately 24 kilometers (15 mi) south of Harbin, to establish a new, much larger facility.[24]

Close-up photo of the Unit 731 main "square building" taken by Unit 731's aviation and photography class in 1940

Other units

In addition to the establishment of Unit 731, the decree also called for the creation 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, Malaya (present-day Malaysia), Indonesia, the Philippines, Papua New Guinea, Thailand, and Burma. All of these units comprised Ishii's network, which, at its height in 1939, oversaw over 10,000 personnel.[25] Medical doctors and professors from Japan were enticed to join Unit 731 both by the rare opportunity to conduct human experimentation and the Army's strong financial backing.[26]

Experiments

The military police and the Special Services Agency were responsible for locating victims to serve as test subjects for the unit, while a group of physicians was tasked with maintaining the health of the victims and dispatching them for experimentation.[17]

The experiments involved intentionally infecting captives, particularly Chinese prisoners of war and civilians, with disease-causing agents and exposing them to bombs designed to disperse infectious substances upon contact with the skin. There are no records indicating any survivors from these experiments; those who did not die from infection were murdered for autopsy analysis.[18] After human experimentations, researchers commonly used either potassium cyanide or chloroform to kill survivors.[27]

According to American historian Sheldon H. Harris:

The Togo Unit employed gruesome tactics to secure specimens of select body organs. If Ishii or one of his co-workers wished to do research on the human brain, then they would order the guards to find them a useful sample. A prisoner would be taken from his cell. Guards would hold him while another guard would smash the victim's head open with an axe. His brain would be extracted off to the pathologist, and then to the crematorium for the usual disposal.[28]

Nakagawa Yonezo, professor emeritus at Osaka University, studied at Kyoto University during the war. While he was there, he watched footage of human experiments and executions from Unit 731. He later testified about the playfulness of the experimenters:[29]

Some of the experiments had nothing to do with advancing the capability of germ warfare, or of medicine. There is such a thing as professional curiosity: 'What would happen if we did such and such?' What medical purpose was served by performing and studying beheadings? None at all. That was just playing around. Professional people, too, like to play.

Prisoners were injected with diseases, disguised as vaccinations,[30] to study their effects. To study the effects of untreated venereal diseases, male and female prisoners were deliberately infected with syphilis and gonorrhea, then studied.[31] A special project, codenamed Maruta, used human beings for experiments. Test subjects were gathered from the surrounding population and sometimes euphemistically referred to as "logs" (丸太, maruta), as in "How many logs fell?" This term originated as a staff joke because the official cover story for the facility, given to local authorities, was that it was a lumber mill. According to a junior uniformed civilian employee of the Imperial Japanese Army working in Unit 731, the project was internally called "Holzklotz", from the German word for log.[32] The corpses of "sacrificed" subjects were disposed of by incineration.[1] Researchers in Unit 731 also published some of their results in peer-reviewed journals, writing as though the research had been conducted on nonhuman primates called "Manchurian monkeys" or "long-tailed monkeys".[33]

At the age of 14, on the encouragement of a former school teacher, Hideo Shimizu joined the fourth group of minors, totalling 35 members, assigned to Unit 731.[34] He recalled that he was brought to a specimen room where jars of various heights, with some reaching the height of an adult, were stored.[34] The jars held body parts from humans preserved in formalin, such as heads and hands.[34] There was also a pregnant woman's body with a large belly, where the lower part was exposed to reveal a fetus with hair. Shimizu discovered that the term "logs" was used dehumanizingly to refer to prisoners. He also learned that the prisoners were further dehumanized by being held in facilities referred to as "log cabins".[34]

Vivisection

Thousands of men, women, children, and infants interned at POW camps were subjected to vivisection, often performed without anesthesia and usually lethal.[1][35] In a video interview, former Unit 731 member Okawa Fukumatsu admitted to having vivisected a pregnant woman.[36] 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.[37]

Prisoners had limbs amputated in order to study blood loss. Limbs removed were sometimes reattached to the opposite side of the victims' bodies. Some prisoners had their stomachs surgically removed and their esophagus reattached to the intestines. Parts of organs, such as the brain, lungs, and liver, were removed from others.[35] Imperial Japanese Army surgeon Ken Yuasa said that practising vivisection on human subjects was widespread even outside Unit 731,[1] estimating that at least 1,000 Japanese personnel were involved in the practice in mainland China.[38] Yuasa said that when he performed vivisections on captives, they were "all for practice rather than for research", and that such practices were "routine" among Japanese doctors stationed in China during the war.[1]

The New York Times interviewed a former member of Unit 731. Insisting on anonymity, the former Japanese medical assistant recounted his first experience in vivisecting a live human being, who had been deliberately infected with the plague, for the purpose of developing "plague bombs" for war.

"The fellow knew that it was over for him, and so he didn't struggle when they led him into the room and tied him down, but when I picked up the scalpel, that's when he began screaming. I cut him open from the chest to the stomach, and he screamed terribly, and his face was all twisted in agony. He made this unimaginable sound, he was screaming so horribly. But then finally he stopped. This was all in a day's work for the surgeons, but it really left an impression on me because it was my first time."[1]

Other sources provided information on usual practice in the Unit for surgeons to stuff a rag (or medical gauze) into the mouth of prisoners before commencing vivisection in order to stifle any screaming.[39]

Biological warfare

Ruins of a boiler building at the Unit 731 bioweapons facility

Unit 731 and its affiliated units (Unit 1644 and Unit 100, among others) were involved in research, development and experimental deployment of epidemic-creating biological weapons in assaults against the Chinese populace (both military and civilian) throughout World War II.[8]

By 1939, Ishii had condensed his laboratory discoveries to six potent pathogens: anthrax, typhoid, paratyphoid, glanders, dysentery, and plague-infected human fleas. These agents were robust enough to ignite epidemics of considerable magnitude and resilient to aerial dispersal. This marked the initiation of the latter phase of Ishii's elaborate scheme: conducting field trials through military expeditions on unsuspecting civilians, aiming to devise a method of dissemination that would efficiently spread the pathogens in optimal concentrations for maximum devastation. His experiments involved developing biodegradable bombs that housed live rats and fleas infected with diseases, designed to explode mid-air, ensuring the safe descent of the infected creatures to the ground. Additionally, he deployed birds and bird feathers contaminated with anthrax from low-flying aircraft.[8] Mice were bred on-site, with 300 rodents distributed from the unit each month.[40]

Plague-infected fleas, bred in the laboratories of Unit 731 and Unit 1644, were spread by low-flying airplanes over Chinese cities, including coastal Ningbo and Changde, Hunan Province, in 1940 and 1941.[41] These operations killed tens of thousands with bubonic plague epidemics. An expedition to Nanjing involved spreading typhoid and paratyphoid germs into the wells, marshes, and houses of the city, as well as infusing them in snacks distributed to locals. Epidemics broke out shortly after, to the elation of many researchers, who concluded that paratyphoid fever was "the most effective" of the pathogens.[42][43]: xii, 173 

The United States Library of Congress holds three declassified documents from Unit 731, each more than 100 pages long, translated from Japanese into English. These documents provided comprehensive clinical records about the daily progression of various pathogens within the bodies of prisoners who were experimented on by Japanese doctors.[44]

Japanese soldiers provided testimony indicating that the research program had the capability to manufacture substantial quantities of biological agents on a monthly basis: 300 kg of plague, 500–700 kg of anthrax, 800–900 kg of typhoid, and 1,000 kg of cholera. Despite the significant production volumes, even small amounts of these bacteria could cause severe harm or death.[45]

Ishii determined that fleas were efficient carriers of the plague, leading Unit 731 to focus on breeding large numbers of fleas. To achieve this goal, Unit 731 had approximately 4500 flea incubators, each capable of producing at least 45 kg of fleas per cycle. The substantial quantities of plague bacteria and fleas generated, combined with the severe illness and death rates associated with plague infection, illustrate the formidable biological warfare production capabilities wielded by the Japanese. Japanese researchers had the necessary materials to apply the scientific method to conduct experiments involving inoculation and the creation of airborne bacterial bombs.[45]

Food items emerged as the preferred delivery mechanism for bacterial transmission. The unit maintained a stock of uncontaminated fruits. In a specific experiment conducted by Unit 731, typhoid was introduced into melons and cantaloupes. Following the contamination process, the bacterial density was measured. Once the density reached a certain level, the infected fruit was distributed to a small group of prisoners, with the objective of spreading typhoid throughout the entire group.[45]

Unit 731 conducted biological warfare field trials by attacking many Chinese civilian populations. During the period of 1940 to 1943, Japanese scientists found that using bacterial bombs for transmission was not effective, but they did find success in utilizing planes to spray microorganisms as a means of biological warfare delivery. Unit 100 also deployed aerial spraying methods akin to those examined by Unit 731.[45]

At least 12 large-scale bioweapon field trials were carried out, and at least 11 Chinese cities were attacked with biological agents. An attack on Changde in 1941 reportedly led to approximately 10,000 biological casualties and 1,700 deaths among ill-prepared Japanese troops, in most cases due to cholera.[46] Japanese researchers performed tests on prisoners with bubonic plague, cholera, smallpox, botulism, and other diseases.[47] This research led to the development of the defoliation bacilli bomb and the flea bomb used to spread bubonic plague.[48] 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, as well as other areas, with anthrax- and plague-carrier fleas, typhoid, cholera, or 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 candy were given to unsuspecting victims. 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.[49] Tularemia was also tested on Chinese civilians.[50]

Due to pressure from numerous accounts of the biowarfare attacks, Chiang Kai-shek sent a delegation of army and foreign medical personnel in November 1941 to document evidence and treat the afflicted. A report on the Japanese use of plague-infected fleas on Changde was made widely available the following year but was not addressed by the Allied Powers until Franklin D. Roosevelt issued a public warning in 1943 condemning the attacks.[51][52]

In December 1944, the Japanese Navy explored the possibility of attacking cities in California with biological weapons, known as Operation PX or Operation Cherry Blossoms at Night. The plan for the attack involved Seiran aircraft launched by Sentoku submarine aircraft carriers upon the West Coast of the United States—specifically, the cities of San Diego, Los Angeles, and San Francisco. The planes would spread weaponized bubonic plague, cholera, typhus, dengue fever, and other pathogens in a biological terror attack upon the population. The submarine crews would infect themselves and run ashore in a suicide mission.[53][54][55][1] Planning for Operation PX was finalized on March 26, 1945, but shelved shortly thereafter due to the strong opposition of Chief of General Staff Yoshijirō Umezu. Umezu later explained his decision as such: "If bacteriological warfare is conducted, it will grow from the dimension of war between Japan and America to an endless battle of humanity against bacteria. Japan will earn the derision of the world."[56]

Weapons testing

Human targets were used to test grenades at various distances and positions. Flamethrowers were tested on people.[57] Victims were also tied to stakes and used as targets to test pathogen-releasing bombs, chemical weapons, shrapnel bombs with varying amounts of fragments, and explosive bombs as well as bayonets and knives.