Korea, etc.

In the unlikely event of an all-out war involving North and South Korea, it's hard to see how China stays on the sidelines. North Korea would eventually be defeated in a confrontation with the United States and South Korea and, with that, U.S. troops would end up in large numbers on China's border. That is as unacceptable to China now as it was in the original Korean War. China might also intervene preemptively in North Korea if it saw dramatic instability, or the real possibility of war.