As crazy as that fat little fuck seems, he knows if he even fires a bullet shot at Guam NK would get leveled with him being the first to have a bunker buster up his ass. Forget Trump, Mattis isn't fucking around
Things are a bit more complicated than that. Even if you assume that South Korea would be ok with a war that would leave Seoul devastated and obliterate large industrial facilities on which South Korea's economy relies, there's another wrench in the works—or rather two:
China and Russia. Do you think China will allow the armistice to end and war to resume? A war that would, inevitably end up with the annexation of North Korea's land by the South? A war in which America would be dropping bombs (including, potentially, tactical nuclear weapons) next to China's borders?
And don't forget Russia. You think they'll stand idle instead of trying to use this situation to their advantage?
Long story short: I very much doubt a war with North Korea will happen for the simple reason that such an outcome is not in anyone's interest. But then again, considering the current leadership on both sides...