~1.3 billion to ~130 million and shrinking. Why fear a 10x smaller neighbor?
You have no idea about the Japanese military. In a one-to-one battle, Japan will beat the Chinese hands down. The Japanese Air Force and Navy is much more superior than that of the PLA. And the Chinese Army, at the best can match their Japanese counterparts. Japan has some of the most advanced weapons in the world, while China produces cheap copycats after stealing Russian technology.
Meh even they are copying everything, Chinese can invade Japan in less than a week, Tokyo unfortunately will not resist too much.
The Japanese after the wwII are against war and I don't think their army alone will get a chance.
No need to talk about the nuke.