in 1950 Mao warned the United States not to cross over the 38th parallel in its counteroffensive against North Korea  and not to advance toward the Yalu river...MacArthur ignored that advice and convinced Truman that Mao was bluffing. 

over half a million Chinese troops entered the Korean War 

China is not going to let the United states decimate North Korea, particularly not through the use of nuclear weapons, the fallout of which would drift down onto China ...nor will China allow the occupation of North Korea by the United States or any western coalition...

That is the reality...there is no pre-emptive military solution to this issue