There's a lot of ruin in a nation. The main axis nations of World War II -- Germany, Italy, and Japan -- provide some examples of nations that were really, really traumatized and damaged. Out of the three, only Germany split apart, and that only because of competing foreign occupiers. Even then it reunited as soon as it got the chance. I don't think there's enough hostility or just plain difference between most of the states west of the Mississippi to cause them to separate, especially under threat of external attack. If anything, I'd expect them to band together as tightly as possible.