North korean missles

Ok i know jack squat about what missles can do what. So, lets just assume they develop a missle capable of reaching our west coast…lets assume they decide to launch said missle. Now, wouldn’t we be able to detect a missle coming actoss the atlantic ocean and just blow it up well off our coastline? What am i missing?

A) you’re behind the times, the DPRK has a missle capable of potentially reaching NY, NY
B) Its a ICBM. That means the DPRK lauches a rocket into space, where is does a partial orbit and then drops back to earth onto its target. It happens very fast, and makes it harder to shot down than a plain Jane cruise missle that would take a long time to ‘fly’ through the air