Unit 731 War Crimes Against Humanity
Japanese Imperial Army's Unit 731 killed thousands of Chinese and Russians held prisoner in Japanese-occupied Manchuria, in experiments to develop chemical and biological weapons.
In the autumn of 1945, MacArthur acceded to granting immunity to members of Unit 731 in exchange for data of research on biological warfare. "The value to the U.S. of Japanese BW data is of such importance to national security as to far outweigh the value accruing from war crimes' prosecution." The BW information obtained from Japanese sources should be retained in 'top secret' intelligence channels and not be employed as war crimes evidence and not be fallen into the Soviet hands. The State Department disagreed over a two year period and the topic simply disappeared.
Why did the US lose interest in pursuing the issue of war criminals? China became communist, Japan was a required base for operations in Korea and became a major trading partner and economic power in the East.