Looking at all the postings, I am mainly inclined to agree with begbert2. But, let us read the question again (something we were told to do when taking an exam): essentially, it concerns what would happen in a civil war in the USA. Assuming it could happen, which I think highly unlikely, it would certainly not be a repeat of the 1861-5 war. There it involved clearly defined issues, and the areas were clearly delineated outside of the states caught in the middle (literally). I don’t see any states trying to secede, still less over any one guiding issue. Unless I am much mistaken, there isn’t one at the moment.
What seems more likely is some sort of local breakdown of the existing structures and extremist groups trying to take advantage of this for their own ends. Looking at this from the outside, I get the feeling that it is only the Reds, the right-wing and mainly white and largely rural groups, that would take action, I don’t really see any corresponding groups among the Blues. One issue that has not been mentioned here, as far as I can tell, is race. Would the main opponents and victims of the Reds be the people of color?
As stated several times, any such civil war would look very much like those we have seen in recent decades, in places like Yugoslavia, Iraq, Syria and Lebanon, and the main casualties would be civilians. The Balkans also highlighted the issue that one of the most destructive factors would be informal and relatively undisciplined irregular or paramilitary groups, mainly very right wing, who tended attack soft targets such as civilians with the aim of clearing out areas through the use of terror. The worst case is a an “everybody against everybody” situation that we have seen all too often in the Third World.
If it is not so much a civil war as an extended riot, albeit with armed gangs on a large scale and without geographical constraints, I think then that this would become a paramilitary police action involve police and soldiers. It would be largely defensive at first, with a view to protecting then population as a whole, and initially at least it would be largely urban. The rural areas would probably just hunker down and see off any outsiders / intruders with a shotgun or better, but not taking any action beyond self-protection. This means that the wild-eyed gun nuts won’t get the level of support they are expecting, those good ol’ boys will just be protecting themselves and their families - and mainly against the gun nuts.
If this hypothetical insurrection is not political, then the military will side with the government. The problem only arises if there is a political crisis in which there is a split, such as happened in 1861, or if the government is not viewed as legitimate. But in the latter case I think then military would be more concerned with maintaining a stable and peaceful situation so that the crisis could be resolved, rather than taking sides.
And the rest of the world would stand back, as happened in the 1860s. If need be, other countries would send in humanitarian aid, but not military aid. As if the USA would need that anyway.
In short, I don’t see that a civil war as such could happen, but perhaps some form of protest that turns increasingly violent, and which would be suppressed by the forces of law and order as a police action. Of course, we have all heard about how militarized the US police are.