so many uninformed responses on this thread.
Kim says shit like this because it is one of the tools he uses to control his own people and look strong internally. I doubt he has any intention of carrying it out. He did not grow up in isolation like the rest of the people in his country did. He knows if he has any chance of survival in any conflict he cannot look like the bad guy. He knows if the US looks to be the aggressor China will pledge their support to him which would likely cause us to de-escalate.
If he goes and nuke something in response to an insult NO ONE will support them and once China backs out he is done.
I agree that Un is posturing, but I severely doubt that the USA would attack first or that China would support NK.
China is one of the biggest forces imposing the new sanctions on NK.
The USA would react to any attack, but our diplomats have repeatedly stressed that we won't start shit. Which is part of why China is glad to commit to the sanctions. NK's time is done. Either the sanctions strangle the life from them, since they aren't self-succient or they take the truly stupid path of attacking someone and everyone jumps their ass.