Since when hasn't North Korea threatened the US? This is nothing new. They have been doing the same shit the last 50 years. Only the last 10 has the nuclear issue came up.
Secondly, techincally we are still at war with them. We are only at a ceasefire. Oh they will declare war on us? Were already at war retard. Try paying attention to current events sometime.
Lastly, negotiations should always be used until last resort has to be applied. In order to win a war in such a situation, you need the backing of many countries to not put it all on our back. Those allies approve of diplomacy before having to be reduced to shooting. So do we. I don't believe anybody should be killed unless they have to be. But if we have to, no fucking around. Finish them off this time. Colin Powell doctrine, massive overwhelming force.
Lastly, maybe just the thing would be to make an example of a country threatening us with a nuclear program. Maybe if just one of them was turned into a field of glass, then the rest of this shit from countries like Iran would stop.