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 sh*tty 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 sh*t 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.
What if the bird will not sing?
Nobunaga answers, "Kill it!"
Hideyoshi answers, "Make it want to sing."
Ieyasu answers, "Wait."
Timelordwho answers "Just as Planned."