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Unit 731 was a covert Imperial Japanese Army program, established in 1936 near Harbin, Manchuria, under the cover name “Epidemic Prevention and Water Purification Department.” Led by physician Shirō Ishii, it conducted lethal experiments on prisoners, mostly Chinese civilians, but also Soviet, Korean, and Allied POWs, including vivisection without anesthesia, deliberate infection with plague and other diseases, frostbite experiments, and live weapons testing. Historians estimate the unit’s broader biological-warfare campaign, including field attacks like the 1940 plague-flea bombing of Ningbo, killed somewhere between 200,000 and 300,000 people.

What makes the case particularly dark in hindsight is what happened after the war: the United States granted Ishii and most of his staff immunity from prosecution in exchange for their research data, and Japan didn’t formally acknowledge the unit’s existence until decades later.