If we indulge the fantasy of a catastrophic civil war, I could see California and New York seceding, and staying out of the mess. Their economies are big enough and independent enough not to need the other states, so self preservation would kick in. California might take Oregon and Washington with them. They already have the “Western States Pact.” - which is only a loose agreement on how to handle Covid 19 restrictions, but it shows they’re working together on some level as a unit. Would NY take NJ and New England with them? I think the real mess would be in the rest of the states. No other country would get involved, they’d be let fight it out. Russia might cast its eye at Alaska, and some distant pacific territory might be tempting to China, if they felt they could take advantage of the situation under the guise of peace keeping.
But look, it’s not going to come to this. A bit of protesting and rioting, at most, and then it’ll fizzle out. Even if it did get worse, the US military is too strong to allow an actual civil war (assuming it doesn’t split into two factions, but it won’t.). Anyone rising up against them would be crushed. This business of a “well armed militia” to guard against an authoritarian government - or anything else - is nonsense. No amount of action men with assault rifles will defend against the national guard, let alone the gear the of the combined army, navy, airforce and marines.