As long as China agrees NK has to go, Trump should not hesitate to annihilate them if they shoot any nukes. Starting a war with NK isn't like going to the Middle East. NK has no allies and is isolated (if China gives their blessing). It wouldn't require any ground troops. A couple of missle strikes to a few of their military installations and the battle is essentially over. They have nowhere to run and would have no way of receiving aid (humanitarian or military). It would be over in less than 72 hours and the world would be much better (and safer).