When North Korea was threatening Hawaii when I was stationed there, we brought in an immense missile warning system and the associated technology to shoot down the missiles. This is what we should do with Guam. This would negate any reasons to pre-emptively strike, and would allow North Korea to make the first move. We would not have to try and justify striking at them first, but rather if they attempted something, it would get shot down (literally), and then we would move in.
I will not be so ignorant as to say a pre-emptive strike can never be justified. But I also know the potential backlash from the world if they do not agree with our justification. Playing the defensive with the anti-missile system does several things. Firstly, it keeps us from having to justify any pre-emptive actions. Second, it gives us automatic justification for handling North Korea if they act. Third, it reveals the bluster of North Korea when the do not act (which I think is very likely).
One of my intelligence officers explained it to me this way: North Korea is a petulant child. Any time the world's focus is not on them, they begin to make threats that they have no intention of carrying out, just to get the world's focus on them again. These threats are similar to an angsty teen claiming they are going to commit suicide, without having any intention of doing so. A "horizontal slicing of the wrist" if you will.
