How Would the U.S. Defend Against a North Korean Nuclear Attack?

I'm not convinced Trump and the US spend a lot of time worrying about how to defend against N.K.

It seems to me Trump would be rather unconcerned if N.K. attacked first as that would then give him the reason he is looking for, the reason to respond in full.

As an outsider it looks a lot like the Richard Nixon’s "madman theory".

As an example Trump, call out Kim Jong In's bluff to attack Guam, where he almost dared N.K. to fire anywhere in the region of Guam.

Kim Jong Un seemed to quickly back down on that Guam threat, only because he was not sure Trump would not have declared war immediately and smashed N.K. with a hail storm of US nukes.

I suspect Kim Jong Un is now thinking, I might be mad but that other guy is truly crazy.