It used to be unthinkable that anyone would take control of commercial aircraft and commit suicide by flying them into very tall buildings.  It seems to be unthinkable that the USA would use nuclear weapons, if attacked.  The island of Guam is American territory and any missiles targeted toward Guam could be taken as an act of war with instant reaction.  If NK Leader Kim orders the firing of those missiles as he has threatened, then the unthinkable may happen again.