Probably nothing. The good news is that Steve Bannon was correct when he did that interview a month or so ago. North Korea already has the capability to kill millions in Seoul so if MAD holds, no one will do anything.

I would bet money that North Korea will not first-strike, and the US military will not fulfill a first-strike if directed.

My premises are that North Korea is a rational actor, and their overwhelming priority is preservation of their regime, and they know without a doubt they can't win a war against South Korea - let alone South Korea and its allies.

I think North Korea will try some scare exercise, mess up, and that's when war will begin. They alone are the authors of their destiny now.

and I think that's what the US secretly hope happens (as perverse as that sounds). so they have a reason to attack

Technically, North Korea fired a missile at Japan and this could have been a good enough reason but wasn't. North Korea could erase life in South Korea, at cost of its own desintegration, but this wouldn't be good to anyone. NK is climbing as higher as it can to get the best deal available, which probably is keep their nuke capacity and the lifting of the sanctions. Other than attacking, I don't know if the US has other option than accepting NK demands.