It depends on where in the trajectory in which they want to shoot it down.
A horror scenario eluded to in a book I read is a typical container ship off NYC that elevates a missile container and launches away. The Norks and Iranitards have the ability to do this. Nothing stops this but intelligence and inspection before such a terror ship gets within range of a major American port.
Thank you for your post. You understand the ballistic trajectory thing. A missile from NK that reaches the US will be moving really fast. Our shoot down capability depends of this speed and at some point we will fail (miss with the shoot down) I am familiar with the THAAD missile and it is more for intermediate range launches. The full scale anti-missile program has not tallied as many “hits: as the THAAD but we are probably working things out as we speak.