Unit 731 was a Japanese research unit tasked with the development of biological weapons for use in the Pacific War. It was the successor to Zhong Ma Fortress, headed by the Imperial Japanese Army scientist Ishii Shiro. The Chinese in Manchuria and other parts of China suffered throughout the war with Japan from germ-ridden packages developed by Unit 731 dropped onto their towns and villages.
After Japan surrendered in 1945, they tried to cover the experiments up but the United States found out. In order to prevent the research falling into the hands of the Soviet Union at the onset of the Cold War, the Americans offered the Japanese perpetrators immunity from their crimes in exchange for the data, to learn from human experimentation such as the US would never countenance. Many were never brought to justice and lived full lives of comfort, wealth and power.