Motive? What possible motive could NK have in threatening to start a war with SK and the U.S.? Unless national insanity is involved, no nation intending to go to war with another country would give that country a warning. NK would like for the world to think they are insane and would do something like commit national suicide, but do they really intend to start a suicidal war with the U.S.?
IMHO, they are just playing a variation of the old “Protection” game and think they just might be able to “shake down” the U.S.
As the U.S. now has a obviously weak President, they just might get away with it.
You forget that NK has allies. By themselves ya no problem. But if China and/or Russia are involved, could get messy really quickly.