It would seem the UN/US's previous attempt to invade is what keeps us from trying it again. That is, fear of a reprisal from China once again. From watching documentaries on the Korean War from the point of China crossing the Yalu River into North Korea onward, it seemed like not one GI would want to face the seemingly endless waves of Chinese Infantry again. We definitely underestimated the motives and preparedness of the Chinese at the time.

Any future action on North Korea without China's approval would definitely not be a good outcome for the UN or the nations involved.