unit 731 was a biological and chemical warfare research and development unit of the imperial japanese army. it engaged in human experimentation during the second sino-japanese war and world war ii. it was responsible for some of the most notorious . 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 it 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.