Meatwad,
I think the book you are thinking of is "Warday" by Whitley Streiber and James Kunetka.
Still have it somewhere......
Yes, in that book, after the nuclear exchange, America and the Soviet Union cease to exist as superpowers, and the other countries step in and "help out" but clearly do not want either the USA or USSR to ever fully recover........and the nation's capital is moved to California, which becomes a closed, or police state.
Interesting reading........Mexico takes back large areas of Texas, New Mexico, Arizona, and California........
Course I may be wrong about the book you are referring to. Back when that book was written, there were scores of fiction novels being written along the same lines of thought.