If there were another civil war in the United States it would be an absolute fuckshow. Both right-wingers and left-wingers are completely delusional about how it would play out.
It would look much less like the first civil war and more like the Syrian Civil War on steroids. Huge swaths of the continent would become ungovernable due to partisans, mercenaries, and the heavily armed population. Terrorist and guerilla tactics would begin to flourish almost immediately; innocent civilians would be targeted, IEDs and probably even dirty bombs would be employed. The interstate highways would be bombed and mined; easy travel across the North American continent would grind to a halt.
The continent would be flooded within months by weapons from any country that has an interest in taking down the United States. This includes not just the usual suspects like Russia and China, but could include Iran as well, and could potentially include countries and parties that one might not expect. Europe, at minimum, would be neutral, and might even supply weapons to one faction or another themselves.
The military would faction and the US' serious war tech would break up. Carrier battle groups would be free agents joining this side or that side. Bombers, fighter aircraft, tanks, and more would split between the various factions and many would be stolen by mercenaries, and either sold for parts or operated as pirate units, services sold to the highest bidder. Some parts of the US Navy might even forsake the country; it's easy to see a destroyer or a submarine going rogue and turning pirate/privateer.
The US' nuclear arsenal is the elephant in the room. Every faction in the war would try to get their hands on as much of it as they could. A kind of fragile MAD would settle over all of North America. The continent would not only be plunged into warfare, it would run the risk of suddenly being nuked by its own missiles. All it would take was one trigger-happy warlord or general.
