Right. NK has a missile that can theoretically so they say reach the States. They have not as yet developed a H bomb there're more damaging than A (fission ) bombs to fit on it. We aren't that sure that their guidance system is that good anyway.
So Kim fires one off towards the states that stands a very good chance of being shot down before it gets too far. They can have anti ballistic missiles stationed in South Korea, on ships in the pacific on Hawaii even so they have a lot of chances to knock it down anyway. They could blow it down a small nuke explosion with a missile passing close would defeat it.
The Israelis have some very good examples of Anti missile tech and I very much expect Americas abilities are kept quiet about, after all they don't wantt the Russkis knowing too much do they?
The damage would be if they had a "Lot" of missiles they could launch at once like the Russians have that's the real problem could they take them all out before they hit the states or anywhere else for that matter.
The Yanks could reply with a bit of Sun-fire of their own, these weapons come in very small sizes up to a few kilotons they arent all big city flatteners.
So it looks to me unlikely that it would be a big exchange and the Russians know that all out exchange would benefit no one hence the MAD principle thats kept the peace for a very long time.
It's in no other countries interests to have a nuke exchange so I expect that Kim will keep up the insults an so with Donald.
It will be more interesting the see what the Chinese influence would be and that outcome NK wouldn't want to lose face so interesting all the same.