This stuff right here:

https://en.wikipedia.org/wiki/Mongol_invasions_of_Japan

A depopulated Japan could easily be an undefended Japan. China is right now working to expand its sphere of territory and influence. They make islands in the ocean where none were before.

This scenario is possible if two things happen: weak Japanese self-defense from depopulation, and a successful closing of US bases by either Japanese citizens who resent their presence, or US isolationism.

Its interesting that the current PM wants to revise their constitution to remove limits on Japan’s military. http://www.wsj.com/articles/japanese-voters-to-render-a-verd...

China is heading towards its own demographic time bomb as well.

There was the one abortive attempt by Kublai Khan with flat bottomed boats which went really sour. If that is considered a chinese invasion is a matter of taste I suppose.

Obviously Mongolia is not part of China, but Kublai Khan did twice try to invade Japan. Typhoons scuppered both chances (and from which the Kamikaze tradition was born).

For most of Japan's history they have had an isolationist stance to the outside world. Obviously this was not the case from between Commander Perry and the end of World War II. With respect to China, for hundreds of years Japan was considered a quasi-vassal state to various Chinese Emperors.