Well, since the US won, they were in charge of sorting out pacific territory. Having minimal vested interest in it they sorta pulled a "you take this, you take that" giving china most of its conquered area back, and giving Japan a bunch of ****** non populated islands. Do note that this isn't the worst territorial divvy up post ww2 in terms of later aggravation.
Now China wants to be recognized as a major power, and the easiest way to do so is to start **** just because you can. And the easiest way to do that without actually fighting is to have random citizens do it for you, rather than the high command.
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Just as Planned.