[Ibby]

A nuclear or massive-scale-conventional assault on North Korea would definitely spark Chinese retaliation, to save face if nothing else. China would feel the need to ensure its interests on the peninsula are kept safe - any solution to the North Korea problem would require Chinese support.

Think of it as, North Korea is China's unruly little brother. China wouldn't necessarily mind someone helping them take care of the rowdy little bugger, as long as they didn't, you know, pound him into oblivion. Kim is embarrassing the Chinese government by throwing his tantrums, but they don't just want him - or, rather, their influence over the region - to just disappear.

China is the biggest player in the region, and ANY solution to the NK problem would need their full support, and to ensure their influence and interests are upheld.

ETA: also, would the U.S.'s interests in Japan or Taiwan or Canada be worth the cost of a war to defend them? Probably not, logically speaking, but treaties and a sense of duty to our friends mean that we would do so anyway. Likewise, China would come to North Korea's aid if they were not reassured that their interests would be looked out for.

ETA 2: Besides, if you were China and detected an ICBM launch towards you, you'd probably shoot first and ask questions later, too.