A new "civil war" would not play out anything like the last one. There won't be any major battles or fighting over territory. What there would be is pockets of resistance where police and military can expect to be shot at, authority defied, etc. Anyone who tries to meet the US military in a real battle is just stupid. The insurrection wins by demoralizing the rest of the country into not fighting. Make it to worth the trouble.
Of course that all really depends on what the casus beli is. You have to realize about 40% of Americans do not love their government. They don't outright hate it either. They tolerate it and it's BS. But it can easily cross a line to make people fight. I always point to gun confiscation as the most likely cause. Because if t ever comes to that then there is no more Constitutional protection of anything. There is no law and what follows will be something I would not want to live through anyway. So for that I'd fight.
I don't see anyone besides overly invested larpers claiming a civil war would be neatly split territory and conventional warfare. A closer model to what it would look like is the Syrian Civil War, where most major cities end up looking like the mysterious Aleppo.
Logistics of insurrection aside, my point is that the US government has been engaging in heavy handed government oppression for at least a century now and that if there were a group willing and able to revolt over this we would have seen it happen a long time ago. If gun confiscation actually happens, and not just to the bad people like antifa and muslims, I'm willing to bet you see a few attacks from scattered patriots and more effort on electing politicans who promise to give the guns back (to the right people) from these supposed rebels.