Unit 731 (Japanese: 731部隊, Hepburn: Nana-san-ichi Butai), officially known as the Manchu Detachment 731 and also referred to as the Kamo Detachment and the Ishii Unit, was a secret research facility operated by the Imperial Japanese Army between 1933 and 1945. It was located in the Pingfang district of Harbin, in the Japanese puppet state of Manchukuo (now part of Northeast China), and maintained multiple branches across mainland China and Southeast Asia. Unit 731 was responsible for large-scale biological and chemical warfare research, as well as lethal human experimentation. The facility was led by General Shirō Ishii and received strong support from the Japanese military. 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 known. An estimated 14,000 people were killed inside the facility itself. In addition, biological weapons developed by Unit 731 caused the deaths of at least 200,000 people in Chinese cities and villages, through deliberate contamination of water supplies, food, and agricultural land. 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 by the United States in exchange for their research data. The Truman administration concealed the unit's crimes and paid stipends to former personnel. However, the obtained information was not of significant value, as the U.S. biological weapons program had surpassed the capabilities of Unit 731. The USSR likely also handed out relatively lenient sentences at the Khabarovsk trials in exchange for information and used Japanese documentation to build its Biological Weapons facility in Sverdlovsk. On 28 August 2002, the Tokyo District Court formally acknowledged that Japan had conducted biological warfare in China and held the state responsible for related deaths.
Formations
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. Japan's occupation of Manchuria began in 1931, after the Japanese invasion of Manchuria. 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. 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. Most of the victims were Chinese, but many victims were also of different nationalities. These facilities contained more than just medical research and experimentation areas; they also included spaces for detaining victims, essentially functioning as a prison. The research and experimentation rooms were constructed around the detention area, allowing researchers to conduct their daily work while monitoring the prisoners. 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. 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. 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. 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. Ishii organized a secret research group, the "Tōgō Unit", for chemical and biological experimentation in Manchuria. 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. 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. 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 Allies suffered 6,000 deaths and 15,000 wounded as a result of the chemical attack.
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