After America would be like the Fall of the Roman Empire, or the collapse of the Chinese Jin (romance of the three kingdoms) and Tang (five dynasties, ten kingdoms) eras, usually because of human bickering over power and control. Occasionally, systems like Shadowrun have a "mild" apocalypse that mostly serves as a catalyst for balkanization. Whatever vestiges of a state remained fall apart under the stress.
Complete apocalypse tends to be something like large scale devastation from a known threat that final gets used (nuclear, biological, dangerous machine sentience) and everybody's too busy dealing with their own issues to care about larger ideas like a continental federal state of "America."
Either way, tends to result in 3+ most of the time. From looking at the Roman Empire and the multiple collapses of China though, it really does not take anything especially dramatic to result in pretty severe balkanization. Often its the old "Blue and the Grey" divide and then most of the West just does their own thing. Occasionally it's more like East Coast, Heartland, and often the West still is not really included.
The result for the West has actually been one of the weirder parts of reading a lot of those settings. Often this undercurrent that the West has never really been a part of "America." The heavily populated East is still mostly fighting over the same issues with each other, the lightly populated West is just some far away land they occasionally pay attention to (mostly California and Texas).
Civil wars and the like are usually based on youth bulges, as they need a lot of breathing bodies to fight it out. Preferrably slightly hungry bodies, as hungry people are easier to provoke into fighting.
One, there are a few counties on Oregon that want to redraw the boundary so that they become part of Idaho. This, I think, is only mildly serious.
The second is the border of Indiana and Illinois, which is serious enough that the Indiana state legislature has voted to create a commission to work on it. It was a bipartisan vote, too. Because there are a number of rural counties in Illinois that would like to join Indiana, and two urban counties in Indiana that say if the option is on the table they’d rather be part of Illinois. Such a thing would need both states to agree and then send it on to Congress, but ultimately I don’t think anything will come of it.
When you look at state funding, these urban counties are sending more tax dollars to their respective state capitols than the states are spending in their counties. In the case of these rural Illinois counties, the state is spending between $5 and $6 per tax dollar collected. Does Indiana really want to take on such welfare queens? And give up some of their few donor counties in exchange? It seems hardly likely!
That’s the rub all across the US. The urbanized areas are subsidizing the rural areas. Are the rural areas prepared to do without such subsidies? They can say “the cities can’t live without the food we grow”, but the entirety of human history shows that the cities always come out ahead in these transactions.
The military have the tanks, the air support, the logistics, the surveilence net, the miscelaneous support equipment, and all the training to use everything.
A split within the military, that gets real ugly real fast.
But yeah, there also was a lot of physically strong young people to choose from.
