What kept the U.S.A. and the U.S.S.R. from starting the "unthinkable", was the strategy called M.A.D. or "Mutually Assured Distruction."
A good idea actually, "Just one missle from 'you', and we all die." If North Korea were to blow every thing they had at us, they might get a state or two, (A couple of million at best.)
On the other hand we could literally remove their half of the penensula from the globe.
The U.S. does not really want to have such a thing on its conscience. The leaders in N. Korea will rely on that "politically correct mindset" and hope that after a couple of their best fifty kiloton "devices" we will be so traumatized that we will just sit in a circle, hold hands and sing three verses of "Kumbaya."
I hope it never happens, as I have forgotten the guitar chords to "Kumbaya."