Actually, unless they really managed to lay a working nuke on a populated area, I think there would be NO CHANCE of a nuclear retaliation on our part, and I don't think it would be necessary. We could easily take them with conventional forces with little loss of American life, and a LOT LESS loss of innocent North Korean life, and this would be highly preferable for a large number of reasons. This may even be true if they DID nuke a populated area. The objective is to destroy their warfighting capacity, not destroy the nation, and unlike a conventional nuclear power with hardened silos and many, many weapons, North Korea may not require a nuclear retaliation to prevent further launches.

We can see where a missile defense would come in REAL HANDY in this kind of scenario.

If they did launch a missile attack, the most likely result would be that they would probably miss the target, and if there was a nuke on board, it would probably be a "dud," which if it hit the ground, would be the equivalent of a small radiological weapon as the conventional explosives on board spread nuclear material around a limited area.