US anti-missile systems are not 100% effective and never will be. The US would not gamble on a missile falling short or flying long, they would try to shoot it down. If they miss an on-target missile and it lands on Guam, I doubt very much that they would launch a counter-attack on Pyongyang because of the danger to its own troops in South Korea and Japan and to the citizens of those countries. They would have to work harder with the Chinese to bring Kim Jong-Un under control and talking about sanctions against China at the very time that that country is cutting down on imports from NK is completely counter-intuitive. Trump is being his usual clueless, pig-headed self.