Unit 731 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 (1937–1945) of World War II. It was responsible for some of the most notorious war crimes carried out by Japan.

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. Others that Soviet forces managed to arrest first were tried at the Khabarovsk War Crime Trials in 1949.

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, as had happened with Nazi researchers in Operation Paperclip. On 6 May 1947, Douglas MacArthur, as Supreme Commander of the Allied Forces, wrote to Washington that “additional data, possibly some statements from General Shiro Ishii probably can be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as ‘War Crimes’ evidence.” Victim accounts were then largely ignored or dismissed in the West as communist propaganda. (read more)