There will be no war involving China or Japan. They both have real militaries, and such countries don't go to war with each other since the advent of WMD. In the modern world, no one is willing to enter a conflict if the other side can seriously fuck them up. China and Japan would be each other's nightmare. On the other hand, China might engage a lesser power if that power doesn't have a defense pact with the US. Vietnam is the only country that China has good reason and capability to go to war with. Right on China's border, wooing foreign factories away, disputing territories, and grossly outmatched with no powerful allies to intervene on its behalf... Vietnam is the country most at risk of a conflict with Beijing. It would be in China's interest to destroy Vietnam's infrastructure as without that they can't compete for the title of most attractive outsourcing host. They can devastate the Vietnamese navy so that the territorial dispute is resolved. As long as the PLA doesn't invade, it'd be a swift victory, a show of force to the world, and a patriotic boost for the discontented Chinese public.