The Japs and Nazi's were unbelievably cruel. It's hard to imagine that they came to power simultaneously.In the book, "Flyboys", there were numerous stories of how the Japs participated in cannibalism, particularly of American POW's. It's no wonder that the Chinese hate the Japs.
It is sort of similar to that continental/island rivalry in western Eurasia (as opposed to eastern Eurasia) with France and Britain (without the Entente Cordiale (spelling?) and the alliances in the two world wars (before Vichy France for WW2).