A serious question

Trump has threatened to rain nuclear fire upon North Korea and the world has great concern because he's unstable and ignorant enough to do it.

However, Trump is a cowardly bully who makes threats and domineering statements all the time. Yet he never follows through. As we see with his phone call with the Mexican president, Trump knows he'll never build his "stupid, fucking wall," (thanks, Vincent's Fox!), and he knows Mexico will not pay for it. In other words, it's an empty threat.

Obviously, nuclear weapons use is far more serious than the wall. But would the cowardly president really pull the trigger? Since I never believe anything he says, I have great difficulty believing he would have the guts to order a nuclear strike.

The question, then, is this: Is Trump's threat believable?