If North Korea attacked and was successful, I think that a lot of things might happen. For one, I imagine that there would be a spike, particularly in centers with large Asian populations, of racially motivated assaults. I don't know what it was like where you're from or where you were living around the 9/11 attacks, but in my home town people were suddenly seeing "suspicious looking A-rabs" everywhere. To compound things, one of Osama bin Laden's many nephews was attending the local university and had to be evacuated for his own safety. I also think that as AlderaanDuran points out, there are a lot of systems in place to prevent a total loss of information. Don't forget that the road system in the U.S. is very good, so there would be little danger of physical isolation occurring.