We should be deploying THAAD up and down the pacific coast and canadian border in the west. The launch is going to come from a submarine.
they have no nuke payload. Everyone can be detected way before they arrive.
Perhaps. Or maybe a rusty freighter - or a suicide boat, or a billion dollar payout for "services" to a drug cartel, or a "gift" to ISIS with a Hertz truck. There are a lot of ways to deliver it if one has the nuke and the will.
The best way to stop it is eliminate it's source.
The NORKS are not able to put a sub off the west coast, even if we were not trying to stop them.