I disagree. Think guerilla warfare, urban warfare, gang warfare, a spreading breaking down of law and order. It won’t be the Posse Comitatas running around the undergrowth in camo, they will be picked off quickly. My guess is that it will be a breakdown of law and order with various groups attacking each other, but necessarily as pitched battles that a conventional military can tackle with overwhelming force. It will be a repeat of Iraq and Afghanistan, or Somalia. The right-wing gun nuts will be on top at first, until they get outgunned and outnumbered
The US military has never been politicised, and I don’t think that will change. The military everywhere tends to be conservative by outlook anyway. It would prefer to do what it knows and does best - killing people - and let the talking heads do the talking.
Outside intervention? Apart from supplying weaponry, I think not. Neither Canada nor Mexico would want to get involved, apart from humanitarian aid, assistance to the police, and taking in refugees. It will be an American matter and will be settled in the USA, simply because of the numbers involved.
Would the states devolve, either before or after such a conflict? I would hazard a guess that some might break away, but would finally return after the whole setup had been renegotiated and Washington got its wings clipped. The return to a USA possibly as a means to restoring law and order nationally.
Big countries can splinter. China has had periods of multiple kingdoms, and the warlord era in the last century. One scenario is for Russia to split up into a number of pieces, some quite small. Mexico is supposed to be at risk of fragmenting. Well, maybe. But we can say for certain that massive civil unrest, whether o not it becomes a full civil war, will be murderous and debilitating.