What preemption against North Korea might look like
...I think China would be deeply involved in handling what is left of North Korea after such a conflict. They certainly do not want American troops at what they consider their doorstep.
But U.S. airstrikes, launched from bases in the South, or from Japan, or even Guam, combined with heavy jamming and cyber ops, could probably severely degrade North Korea’s ability to launch anything. Their internal communications would almost stop. And if you are going preemptive, there’s a good chance you are going to conduct a decapitation strike aimed at destroying Pyongyang’s ruling elites. Drones could loiter overhead to provide warning of any launcher rolling out of a tunnel. Being a North Korean launcher operator would become the world’s most hazardous job, but with plenty of opportunity to move up the ladder.
Here’s the first big problem with preemption: North Korean artillery. They have a lot — I mean, thousands — of artillery pieces along the DMZ. And the moment they got a whiff of what was going on, they’d probably start firing shells into Seoul. I don’t think they could keep it up for long — the tubes are probably shoddy, they’d have a lot of duds, and they’d find that keeping them firing is hard when the other side is firing back and probably cluster bombing every tunnel door that opens. The prospect of artillery shells landing around the greater Seoul area, with a population of over 20 million, is grim, even if the shelling can’t reach the southern part of the city, and even if it lasts only a few days. But it still wouldn’t be the “sea of fire” that Pyongyang a few years ago promised to rain down on Seoul.
And then there’s the Pottery Barn problem: The more you remove the Kim family monarchy in the North, the more you own the problem — and are responsible for, among other things, feeding and policing the population. Even if you can persuade the UN to put in peacekeepers, the United States is going to have to provide the logistics — few other nations are capable of doing even a fraction of it. There almost certainly would be major humanitarian relief operation, at the very least.
And what if not everyone in North Korea is happy with this turn of events? I think most of them would be delighted to see the absolutist Communist monarchy gone. But some will not. I remember being told that U.S. military planners were especially concerned by the northeastern part of the North, up near Russia. What if that chunk held out, declined to agree to a cease fire, and insisted it was carrying on the great traditions of the Kim family? Then you are going to need more than peacekeepers. And we all know what the warnings about getting bogged down in a land war in Asia are.