Unit 731 was created in 1936 by the authorization of Emperor Hirohito in Japanese-occupied Manchuria with the aim of developing biological weapons. It was led by Ishii Shiro and had a partner unit, Unit 100. Unit 731 was supported by Japanese universities and medical schools which supplied doctors and research staff. Organized under the alias of The Epidemic Prevention and Water Purification Department of the Kwantung Army, Unit 731 operated as a covert chemical and bacteriological warfare research and development unit, conducting and responsible for some of the most atrocious war crimes of imperial Japan.
According to several former workers of Unit 731 and Unit 100, the laboratories were constructed for the purpose of manufacturing bacteriological weapons, which the Japanese Army would use against namely the Soviet Union, Mongolia, and China. While the units were not created with the sole purpose of preparing Japan for war against the USSR, there is evidence that proves the expected war was one of the major motivations.