honestly, with the amount of tactical insertion subs that the norks have lying around, it wouldn't surprise me if these purported nuclear weapons were actually inserted into the US a coastal landing, moved to the target by ground teams, and detonated on the surface. That way, it could be pawned off as terrorist attacks and the north has no blood on its hands diplomatically.
This means our missile defense is worthless, makes homeland security look like a bunch of morons, and could (possibly) scare the US out of a war.
Or, it could steel the resolve of the population and make a war actually happen.
edited for clarity