“Can the US Defend Against a North Korean Missile Attack”, by Warthog Defense, explains the GMD (Ground Missile Defense) strategy against a low-volume nuclear attack that could happen before 2020.
GMD is predicated on destroying a missile as it re-entry with the kinetic energy of a collision.
The film confirms that Bush was concerned about the idea of an eventual North Korea attack as early as 2002.
A similar defense from NORAD should deflect any high altitude launch from a terror group from offshore intended to launch an EMP attack with a small high-altitude nuclear explosion or possibly flux device.
Check out Foreign Affairs (paywall), Jeffrey Lewis article, "Kin Kong Un's Quest for an ICBM, the State of North Korea's Missile Program", here.