So, you want the military to carry through ILLEGAL orders given by a president?
Well, we just lived through 8-years an illegal President giving many illegal orders. ("illegal"="unconstitutional").
In terms of the President's Constitutional responsibilities, the question here comes down to whether he determines, with clear evidence, that NK must be preempted.
The troll neverTrumpers here and the demoncrats everywhere are trying to imply DT would OR could nuke NK on a whim. Sheer BS.
The only legitimate "whim" target for a nuke might be DC.