I don't think any of the factions in the US are going to be regionalized enough to make a war with borders and territory. Instead there'll be a lot of parallel, unrelated conflicts, happening more often and occasionally overlapping. So the police vs BLM clashes of last year, the battle of Blair Mountain, white supremacist terrorist attacks against governing buildings (last year again lol), all that stuff happening at once. There's a bit of city vs countryside in that, but it's not 100% consistent, and neither of those groups has the ability to take and hold territory, so it can't really become a territory dispute.

That'll all continue onwards while the federal government does less, city governments do more, and state governments get used as proxies by their most powerful cities. The federal government won't officially collapse, it'll just do less and less until some group gathers enough power to do something on a national scale again. Then some leader from that group gets elected president, and the official structures have continuity so we can pretend the collapse didn't actually happen.