I think Asia is pretty finely balanced at this moment. China, Japan, India and Russia are great powers and fairly evenly balanced. Even countries like Vietnam , South Korea, and Australia wouldn't be pushovers in an invasion; especially with the latter two's close ties to the US. There are no fully fledged alliances between any of these countries like in cold war Europe against Russia but they all watch each other carefully for signs of aggression. If any one nation made threatening moves, the others would likely band together against it.
As for Japan specifically; the one thing that would most likely set them off would be further nuclear threats from North Korea. Already it seems like perhaps they are using their military role in the Iraq coalition to address the constitutional issues and strengthen their stance towards North Korea.
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