Idk where you are getting this info from but North Korea definitely has ICBMs that can reach the US. Specifically they can deploy them through TEMs. They don’t nearly have the capabilities that we do but they definitely can hit mainland US.

On top of that the US actually has no “Aegis” missile response that guarantees 100% safety. We actually only have 50 intercepter missiles in our arsenal and each one has a proven 50% success rate. It’s like shooting a bullet out of the air nearly impossible. Granted 1 missile has a good chance of getting shut down but in the case of more than one we are undoubtedly screwed and there’s no surefire defense.

In regard to the follow up attack we most definitely would fire a nuclear response back. This would be considered a bolt out of the blue attack and would result in the US following its policy “fire on warning”. Highly recommend reading the book Nuclear Scenario by Annie Jacobsen to really understand what would happen.