I take these statements from Mattis in such a way that hopefully there are at least two steps before the hot war starts, not one step. I.e., if fat Un really launches missiles towards Guam explicitly warning in advance that the target area is somewhere in neutral waters, then it is not yet a war (though a violation, ignorance, etc. etc). US would (hopefully) intercept whatever NK launches (I assume there would be some light imitators of real warheads as they are unlikely to reach such a technological level so quick) but not necessarily strike massively the NK territory. IMHO such a successful intercept would put the fat one into sh..t up to his ears, and this would be a good thing. Would it stop him from further development, sure not. More sanctions (even from China), very likely yes. But though a threat for Seoul would not become less, it would be a great relief for Tokyo, Hawaii, Guam and continental US.