To make a long story short, I strongly believe that any war between the United States and North Korea would result in the collapse of North Korea’s government and a massive flood of refugees into China and South Korea. This is likely the reason (along with North Korea’s nuclear weapons) why the US has not toppled the North Korean government yet.
In fact, it’s very likely that behind the scenes both South Korea and China have been sending North Korea subtle signals that regime change is not a desired option. This would explain why the North Korean government hasn’t collapsed considering that it doesn’t and hasn’t been meeting the needs of its people.
Who would win such war?
It all depends on what you mean by “winning”, the circumstances in which the conflict begins, and whether North Korea uses nukes/WMDs. If we ever engaged North Korea, I don’t see the U.S. using nukes because our conventional weapons are effective enough.
First, discussing the circumstances, if North Korea attacked the South, it would take longer than if the South/U.S. attacked North Korea. The reason is that you would have to stop their offensive before moving back north, account for casualties/battle damage, etc. If the South/U.S. attacked the North, we would be able to neutralize a lot of their military power right off the bat, which would make things go much more quickly.
If intelligence allows for forces to mobilize early before the conflict begins, it would be faster than if they attacked by surprise and we had to spin up logistics.
If North Korea used nukes or WMDs, it would significantly complicate things both militarily and politically. It could be slower due to significant damage to our forces, or it could be much faster due to the rest of the world coming together to “take care of” North Korea.
One important factor… drones. Drones can indeed make their carefully dug in positions very vulnerable, and judging by how they are building up their military, that’s a big problem for them. I think we would also easily maintain air superiority. Given that it’s a small, extremely mountainous country, it would seem to be relatively easy to use drones and air strikes to completely disrupt their supply lines. There simply aren’t that many places where rail lines or significant roads can run, especially across the river. And we have become very good at using drones, helicopters and so forth to spot units moving and take them out from a distance.
I think there is a not insignificant possibility of North Korean collapse once the war goes on for any length of time. They may have many men under arms, but they cannot feed them in peacetime, much less in war. If they can’t accomplish a blitzkrieg and get at South Korea’s food supplies they will quickly run out of food. They also have very limited supplies of other things because they have been at famine levels for years. Outside of a few zones where they cooperate with the South (and maybe China) their industrial base is small and aged. They have few natural resources and even now have less ability to exploit them at the level that a modern industrial nation can.