But would the west stand idly by while Japan basically pulls a Nanking on all of China? If not out of humanitarian concern, then surely out of economic and geopolitical interest, they wouldn’t want an entirely subjugated China to any one power, much less to one aggressively expanding like Japan.
Apart from that, the Japanese War Machine run on oil, and there wasn’t much of t. Sooner rather than later, Japan would run out, and would be unable to conduct a war in a China, unless they invaded places where there was a lot, at which point you come into conflict with European colonial powers in Asia and eventually the US, essentially getting OTL.
But even then, would it really be in Japan’s interest to try to subjugate and pacify such a vast area, population, a fairly different civilization, when the tide of decolonization would effectively nullify all that in three, maximum four decades?
France couldn’t manage to keep he lid on Algeria, a colony that had a sizable french population, where French and Arabic were both main languages rather than Arabic, with French as the language of prestige and administration, it had a much smaller population and still it got away. Thus, I think that Japan has no chance to conquer China, and keep it for more than half a decade.