Losing CA alone would make the US possibly not even a world power. NY and CA gone, the US Would be a thing no one really has a reason to think of talk about. Suddenly all the movies would be about aliens landing on Canada or blowing up Canadian national landmarks.
It's never worked and never will, that's the whole point of the Civil War. The military wouldn't just sit back and do nothing, and states that try to leave wouldn't have a professionally outfitted military. My opinion, if anyone really tried, a few handcuffs would end their attempts.