If you allow me to interject, as of September 7, President Trump issued a shoot-down order for any missile heading towards the United States. I don't have the specifics of the order yet, but my understanding of the order is that it will be shot down during what's called “boost phase”—that is, literally, when it flies from North Korea at the very early stages—which means you're doing it with missiles based in Japan on vessels or on land. You're not even going to give them a chance to get to Alaska, get over the horizon, because you can't take a chance. A shoot-down at boost phase, if you think about it, is incredibly provocative. They have concluded they could not take the chance of a surprise attack. They have concluded that North Korea is likely to do a surprise attack.
On September 14th, 2017. See this statement in context.