American cover-up of Japanese war crimes

The American cover-up of Japanese war crimes occurred after the end of World War II, when the occupying US government granted political immunity to military personnel who had engaged in human experimentation and other crimes against humanity, predominantly in mainland China. The pardon of Japanese war criminals, among whom were Unit 731's commanding officers General Shiro Ishii and General Masaji Kitano, was overseen by General of the Army Douglas MacArthur in September 1945. While a series of war tribunals and trials was organized, many of the high-ranking officials and doctors who devised and respectively performed the experiments were pardoned and never brought to justice. As many as 12,000 people, most of them Chinese, died in Unit 731 alone and many more died in other facilities, such as Unit 100 and in field experiments throughout Manchuria.

Historical background
Negative sentiments had existed historically between Japan and China, but those rarely materialized in armed conflict prior to the early 1930s. Japanese militarism under Emperor Hirohito had escalated from the Mukden incident in September 1931, when members of the Japanese Imperial Army planted a bomb on a Japanese-owned railway track in Manchuria. They used this "attack" as a justification for a full-scale invasion of Manchuria and the establishment of the Manchukuo state. The immediate predecessor events of the Manchurian invasion include the abovementioned Mukden Incident and the preceding Wanpaoshan Incident in July 1931, which constituted a dispute between Chinese and Korean farmers in Manchuria. The build-up to the invasion was minimal and involved no deaths on either side of the conflict. The US was mostly unaware of the events that occurred outside the main Pacific theatre, which gave the Japanese army, headed by the Kwantung Army, political and military freedom to take advantage of a technologically and strategically inferior China. American propaganda depicted Japanese militants as sub-human animals, not unlike rodents or snakes. The perception of the Japanese soldier in the US during the war was that of a lowly, depraved and inherently repulsive being, whose depictions in propaganda pieces varied. They were often depicted as animals traditionally associated with depravity and as such intuitively instilled disgust.

Cover-up
The American government sent General MacArthur to oversee rebuilding post-war Japan and the shift to a democracy from a previously feudalist system of governance. MacArthur was also responsible for gathering data on biological warfare, which was obtained through human experimentation. The US government offered full political immunity to high-ranking officials who were instrumental in perpetuating crimes against humanity. Among those was Shiro Ishii, the commander of Unit 731. Hirohito, as emperor, gave his consent regarding the policies and activities of Unit 731, Unit 100 and other human experimentation facilities. He supported the militant agenda, but there is no evidence that he was thoroughly informed on most of the atrocities that occurred within the facilities. He, as was the case with many others, was also granted immunity. MacArthur, abiding by the Potsdam Declaration, gathered a jury for the Tokyo trials, where a number of Japanese officials were successfully tried and convicted.