They have been accused of conducting a series of human rights abuses against civilians and prisoners of war (POWs) throughout east Asia and the western Pacific region.
The empire also violated provisions of the Treaty of Versailles, such as article 171, which outlawed the use of poison gas (chemical weapons), and other international agreements signed by Japan, such as the Hague Conventions of 1899 and 1907, which protect prisoners of war (POWs).
According to historian Akira Fujiwara, Hirohito personally ratified on 5 August 1937 a proposition by his Army chief of staff, Prince Kan'in Kotohito, to remove the constraint of those conventions on the treatment of Chinese prisoners.
However, any convictions for such crimes are not required to be recognized by the Japanese government, as the Kellogg-Briand Pact did not have an enforcement clause stipulating penalties in the event of violation.
Under normal circumstances, it violates a number of fundamental principles of modern legal procedure to punish someone whose crime and penalty were defined only after the fact.
Had Japan certified the legal validity of the war crimes tribunals in the San Francisco Treaty, this might have resulted in Japanese courts reversing such verdicts.
By comparison, the United States did not come into military conflict with Japan until 1941, and thus Americans may consider "Japanese war crimes" as encompassing only those events that occurred from 1941 to 1945.
A complicating factor is that a minority of people in every Asian and Pacific country invaded by Japan collaborated with the Japanese military, or even served in it, for a wide variety of reasons, such as economic hardship, coercion, or antipathy to other imperialist powers.
The Indian National Army, under Subhas Chandra Bose, is perhaps the best-known example of a movement opposed to European imperialism, which was formed during World War II to assist the Japanese military.
The Burmese nationalist leader Aung San initially sided with the Japanese, forming the Burma National Army, but turned against them in early 1945.
B. V. A. Roling, the Dutch justice at the Tokyo trials, noted how "many of the commanders and guards in POW camps were Koreans [as] the Japanese apparently did not trust them as soldiers."