They have the "Japanese Self Defense Force" and after 9-11 they amended their Constitution to allow it to serve outside Japan so they could assist us in the war on terror, but while they are very professional the force is very small, and aimed primarily at SELF defense and Fisheries protection, while N Korea has one of the largest Armies in Asia, and when they test their missiles they often shoot them over Japan into the Pacific. Japan has the lead in quality, but they would be swamped by sheer numbers.
China ? As far as Taiwan/ Formosa it's hard to say because so much of the decision would depend on internal Chinese politics, as for the rest of the region I see them preferring a course of economic domination, and political influence rather than military domination, although the military option will always have a place in theirs neighbors minds.