If North Korea launches a missile at us, it's either gonna end up in the ocean or blown up in their faces. At which point we can just loving steamroll them.
Some people are talking like this would be a war like Iraq, which is completely wrong. This would be a war like the Gulf War, where we forgeted up the Iraqis big time with only double-digit casualties. It'd likely be over within a couple of days. We are the most powerful military in the world bar none, plus we would presumably have the backing of NATO and anyone who felt threatened that NK actually fulfilled one of their promises. NK is a speck of dust. They don't even have widespread electricity in their country. They have no chance of winning whatsoever, even if they do a Killzone 2 and detonate a nuke in their own city like madmen.
Once we win, just do what we did with Japan and Germany, in coalition with SK, and we're all set. It'd be like removing a wart.