On paper Japan looked like a quasi-democratic constitutional monarchy until the militarists completely grabbed power in the late 1930's. Unfortunately, it was an aggressive and expansioninst quasi-democratic constitutional monarchy.
9-Fang's point is well-taken. Nations ally with others based on perceived threats to their interests, not politics. Nothing in 1930's nationalist Chinese foreign policy indicated it was a real threat to western interests in Asia - even in China - given the extent the Chinese government continued to tolerate the European enclaves. Japan, on the other hand was making noises and taking actions that directly threatened US interests and, less so, those of the British, French, and Dutch. Plus, once the Sino-Japanese war began in earnest, documented brutalities of the Japanese army would have made it politically impossible for the western democracies to ally with Japan ... unless for some reason the USSR formally enters the war in the mid-late 1930's to ally with the Reds and invade Japanese occupied portions if China. Maybe, maybe, if Japan avoids joining the Axis and distances itself from Hitler in particular, and plays the victim of naked Soviet aggression - coupled with the real risk Manchuria and Korea might fall into Communist hands - the Western allies might at least consider the Japanese as some sort special case when or if WW2 in Europe starts: not an ally, but not an enemy either (sort of the Soviet/Japanese non-aggression pact in reverse), and even provide them some aid and trade.