It has already started; a feeling like there is a difference between us and them. That feeling goes both ways-Californians feel that they have little in common with Alabamians, for example. This creates factions, but they have to be factions that are generally divided along both ideological and geographical lines. Then there's an instigating event. Something that is so onerous that it makes one of the largest factions leave. I don't know what that would look like. The other factions choose to let that faction go, or only put up a mediocre resistance, or cannot stop the other faction from leaving. Then you get the 1947 Partition. Maybe with bloodshed, maybe without, but you will see a mass migration of people as they move to be with their own ilk. The US breaks apart, and several smaller, regional countries emerge. Broad brushstrokes.