When push comes to shove, N. Korea may be in a two-front war. PLA coming from north, S. Korean and U.S. coming from south. Each group in a race to reach Pyongyang first. I am not kidding.
We might be wise, after total, massive shock/awe aerial bombardment for 48 hours, stage an Incheon-type west coast landing (combined with massive air drops), that is, 1/2 way up the North Korean coast, somewhere north of Sunan, and attack/move both to the east and then south to take Pyongyang, and to the north to hold the line against advancing PLA coming across the Yalu. They will have some of the northern half of the peninsula, but we must limit their occupation to as little an area as possible. As we divy up the spoils, we run the huge risk of a direct PRC-USA head on clash.
If China decided they wanted to, when prodded Kim Jong-il would let them move down to the DMZ before we decided what to do about it.
It's a close call, but I think China will let the South have it. They have nothing worthwhile to gain from taking the North.
Sounds like a pretty short war to me. But what would realistically cause China to invade NoKorea? Would Kim really wave his missiles at China?