system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the US was attacked with a long-ranged ballistic nuke by a foreign nation, would we be equipped to disarm it before landfall and how does this technology work?
gpt
The current belief is (hopefully never to be fully tested) that a few missiles could potentially be stopped.  Depending on from where it's launched, the US has a few layers of defenses posed to try and stop it, primarily with missiles designed to intercept other missiles.  We've seen this technology improving and in action in places like Israel against short range ballistic missiles and other weapons.  Against large numbers of targets and at high altitude the reliability understandably decreases, not taking into account special measures/technologies that might be employed to defeat interception missiles.

The biggest defense against a nuclear attack is the concept of Mutually Assured Destruction, which is to say that if we detect an attack we're ready to respond in kind before any warheads actually impact, leading to significant destruction on both sides.  No rational power would challenge such a threat.  Even a state like North Korea, which really acts the way it does solely to preserve its particular political structure.