I say the same thing as I said about Taiwan, what the heck China? You control over 1/5th of the world's populace and yet you greed for more. Not to mention, Japan is a freer country then you.
Isn't Japan a Non-NATO ally of the US (I don't for sure remember.) Perhaps we could use it as an excuse to threaten China to back out of the NK situation (Just let us free them already) and to stay the heck out of Taiwan. Otherwise, threaten invasion. They wouldn't call the bluff if we had a strong and bold enough man to do it.