They don’t have a military. The National Guard units would come under the command of the President of the US, and any units in rebellion would know they’re facing courts martial for crimes that would be career limiting in that the penalties could include anything from life in prison to execution. It’s literally treason by the legal definition.
Even if any significant number of troops were to choose to violate the law, modern war isn’t about riflemen. There’s a massive infrastructure required to keep tanks and planes running, not to mention things like carrier battle groups. Northrop and Raytheon aren’t going to be forfeiting USG contracts to sell missile systems to Ohio.
Only the president has the nuclear codes, so nuclear blackmail can’t work either.

There isn’t going to be another civil war. Too much has changed between then and now in terms of military and economic organization. This is just Texas whacking off yet again, as they did under Obama and Bill Clinton.

The very real risks we’re facing are the election of Donald Trump - this is the biggest threat - and far right domestic terrorism. The former is an existential threat to the United States and should be treated as such. The latter is a law enforcement issue and should be treated as such. I suspect the Proud Boys are infiltrated all to hell as are the other major organizations, but there’s the potential for a significant amount of harm being done on a larger than 9/11 scale, although it’d be drawn out.