If  a war begins, the US will conduct a  massive preemptive attack targeting command and control  and will do everything possible to decapitate  the North Korean leadership.  It is not likely that nuclear weapons will be used although  low yield neutron bombs could be used to eliminate North  Korean forces near the DMZ to reduce their ability to retaliate on Seoul and neighboring areasof South Korea.  It is very unlikely that South Korean and US forces will invade.  The focus will be to eliminate and or isolate much of the ruling military elite and let chaos ensue.  The ChiComs may send in  troops but their presence would not be welcome by the North Koreans and would be unwise. The better option for the ChiComs  would be to provide food and medical aid help and police forces to help stabilize things afters the dust settles.   The eventual goal should be for a reunification of  Korea.   Once this occurs, the US will be able to withdraw its forces and the Korean War will finally end.