Unit 731 was a biological and chemical warfare research and development unit of the Imperial Japanese Army. It engaged in lethal human experimentation during the Second Sino-Japanese War (World War II). It was responsible for some of the most notorious war crimes committed by the armed forces of Imperial Japan. Unit 731 was based in the Pingfang district of Harbin, the largest city in the Japanese puppet state of Manchukuo until 1945 when the Soviet Union invaded in what would be known as The Soviet Invasion Of Manchuria Of 1945. It was active throughout China and Southeast Asia. Estimates of those killed by Unit 731 and its related programs range up to half a million people killed on the orders of the camp's director Shirō Ishii.

It was officially known as the Epidemic Prevention and Water Purification Department of the Kwantung Army. Originally set up by the military police of the Empire of Japan, Unit 731 was taken over and commanded until the end of the war by General Shirō Ishii, a combat medic officer in the Kwantung Army. The program received generous support from the Japanese government until the end of the war in 1945. Unit 731 and other units operated biological weapon production, testing, deployment, and storage facilities. They routinely conducted tests on human beings (who were internally referred to as "logs"). Additionally, biological weapons were tested in the field on cities and towns in China.

While Unit 731 researchers arrested by Soviet forces were put on trial, those captured by the United States were secretly given immunity in exchange for the data gathered during their experiments.^[1] The Americans coopted the researchers' bioweapons information and experience for use in their own biological warfare program, much as they had done with German researchers in Operation Paperclip. ^[2]