This is the classic "Art of the Deal" Trump modus operandi. Unfortunately for us all, this isn't some New York real estate deal. 

Happily, the danger isn't really a nuclear war (although one can't totally rule this out) but rather that he is playing chicken with a regime that can't afford to lose.  Rather than forcing NK into a corner & into backing down, Trump has walked into something of a corner himself. There will be an inconclusive standoff whereby the US loses face with its SK, the Philippines & Japanese allies & with China & Russia.

It is the longer term (rather than the immediate) implications of this that are most worrying.