Funny but this very question came up in post WW-ll war crimes prosecutions in Asia.
The Americans, specifically Douglas MacArthur, wanted to avoid any prosecutions of the Japanese royal family even tho they were guilty as sin. Most of all the Emperor and his uncle, the one who commanded at the rape of Nanking. The reasons for this was Political expediency. MacArthur wanted the cooperation of Hirohito for his occupation forces and most of all the adaptation of his new constitution. MacArthur rightly saw Japan as the new front line in the next war with communism and he wanted internal stability in the new Japanese Democracy. To the Japanese Hirohito was the equivalent of Jesus to Christians and without his help MacArthur knew he would fail in the reconstruction of Asia.
One of the reasons he used for not prosecuting war crimes for the most heinous Japanese actions, the ones during their rape of Asia, was that it occurred during a "different war" and that WW-ll didn't actually start until 1 Sept. '39. So there was no trials for Nanking, the rape of Korea, the comfort woman, or even the chem/bio experiments of unit 731, "we wanted their records and cooperation for weapons development". MacArthur's reasoning was that these crimes happened during a "different war, one between ancient Asian peoples". The whole episode was a shameful chapter in American history. Even if it was a necessary one.
But if you asked 100 Historians when WW-ll started I bet most would say 1 Sept. '39. During the period right after the conflict I'd say 99 out of 100 would have said that.