How Can the US Defend Itself From A North Korean ICBM Attack?

That is what the National Interest answers for its readers:

But as with almost all North Korean developments and proclamations from the exalted “supreme leader” Kim Jong-un, it’s hard to say what’s real and what’s a delusional fantasy. Nonetheless, Washington has to take the possibility of a nuclear-tipped North Korean ICBM seriously. What, then, can the United States do to stop a North Korean ICBM that’s headed towards the American homeland?

To a large extent the United States has prepared from this eventuality. The best way to stop a North Korean ICBM would be to eliminate the target before it’s launched. But if the regime in Pyongyang—in a move that would guarantee a devastating U.S. nuclear retaliation—manages to launch a missile towards an American city, Washington has an operational missile defense system in place.

The Missile Defense Agency’s Ground-based Midcourse Defense (GMD) element provides Washington the ability to destroy a limited number of incoming intermediate and long-range ballistic missiles threats in space to defend the United States. There are interceptors already in place at Fort Greely, Alaska, and at Vandenberg Air Force Base, California. Right now there are a total of thirty interceptors deployed, but there are plans to add fourteen more missiles.   [National Interest]

You can read more at the link about other missile defenses the US has, but first of all launching an ICBM at the US would be a weapon of last resort since the US response would be so devastating against the North Koreans.  Additionally the rocket they are threatening to fire now would be easy to destroy left of launch since it takes days to prepare it for launch.  That is why the North Koreans have fielded the KN-08 ICBM that is road mobile and much more easier to launch.  However, it has never been tested and many have claimed they are mock ups.

For all we know the Kim regime could be playing the same games that Saddam Hussein did with exaggerating his military capability.  However, military leaders have to prepare for the worst case a threat poses and thus why the Kim regime’s threats are taken so seriously.