As Supreme Commander for the Allied Powers , MacArthur also decided not to prosecute Shiro Ishii and all members of the bacteriological research units in exchange for germ warfare data based on human experimentation . On May 6 , 1947 , he wrote to Washington that " additional data , possibly some statements from Ishii probably can be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as " War Crimes " evidence . " The deal was concluded in 1948
bacteriological research units
Unit 731
Supreme Commander for the Allied Powers