I always find it hilarious that people think the second amendment would matter much in a US civil war (or whatever internal conflict you want to imagine).
If the US military is united behind one group then that's that. If the US military is divided, then god help us caught in the middle.
Yeah, a non-divided military + police usually means a very short and decisive civil war, in observable modern cases. The exceptions tend to involve a divided armed forces, or extensive foreign interference on behalf of the rebels (see: Syria).
For some reason, folks like to cite US foreign intervention failures as proof motivated locals with rifles can beat the US military, but that's not really the right thing to look at, as a bunch of things about those situations are materially different from a civil war (plus there is in every case a ton more to the resistance's armament and materiel than some guys taking their old AKs out of the closet, dusting them off, and digging into their prepper-crates of MREs)
Consider insurgency as a possible way a civil war would play out. Asymmetrical wars (or Small Wars) are very hard for conventional armies to fight. And even harder to win.
If the US military is united behind one group then that's that. If the US military is divided, then god help us caught in the middle.