Over the course of its long history, China has engaged - as one might expect of just about every country in the world, over time - in border clashes with all of its neighbors. The clashes where victory came at a low cost were exploited to the hilt, such that new provinces were added to the empire. During the present regime's rule alone, China has clashed with the Soviets, the Indians and the Vietnamese, and annexed both Tibet and East Turkistan (now Xinjiang). China's rulers are rational players - cost-benefit is the key consideration. The Korean War was a bloodbath for the Chinese (1m dead) because Mao ruled the country like living deity - worshiped by all except his closest compatriots - and even that war's casualties paled relative to historical Chinese losses (where anywhere from 1/3 to 1/2 of the entire population died, a number typical of medieval era wars everywhere, due to famine and famine-related epidemics). As far as the Chinese are concerned, god is dead, and he's embalmed in a mausoleum located at Tiananmen Square. Now that Mao, the Chinese deity, has expired, China's uncle point is much lower than the 1m dead of the Korean War.
Interesting. Given the current administration, I would say our uncle point is far lower than theirs. And, everybody knows it.
Getting all those other countries involved makes this not just between Japan and China.
Much more likely that China goes no farther now that it knows Japan has a lot of back up.