how would it go? warn them on late on a sunday evening and then hit their nuke manufacturing facilities one hour later with blockbusters?
Right now they have all the warning that they will ever get. If we strike North Korea it will be a massive attack designed to destroy all of their military power in the first blow.
The last thing that you want to do is give that crowd in Pyongyang a chance to destroy Seoul or Tokyo in response getting hit. My guess is that we have been offering Kim a way to surrender while saving face. Kim appears to prefer getting slaughtered.