Hmm....
I think the PLA could own the US, to tell the truth. China has always have numbers in everything. They could have sent human waves of soldiers and forced us to pullback, or something like it. WW3? Maybe, maybe not. If it turns really bad, the A-bomb would have been use in Korea, or even China itself, but you have Russia to think about. Stalin might use this to try and move in on Europe. Of cousin, he knew of the bomb, so he might just do nothing and let Asia got hit by A-bombs.
Afterwards, it would ruin the US-PRC relationships that came after the war in the face of the USSR.
A divide Korea might be even better. At least the South or North would be far better then the very corrupt whole Korea nation we have today