I honestly don't think they could even if they wanted to, in this day and age where everybody can take and send video. They first couple times a video leaked of them shelling a school and killing women and children (either by accident or intentionally) there would be a huge backlash not only in the world community by even in the "northern" states they rely on to support the war to end the hostilities and negotiate.
Not only that, but a Civil War II would look nothing like 1861 to 1865 with nice orderly lines of uniformed soldiers marching around, it would look more like the Libyan/Syrian Civil war or even the Iraqi Insurgency, where the combatants can ambush a convoy or base and then blend in with the populace.
I would hope that the fools supporting insurrection would not shell school, but they certainly are hotbeds of liberal indoctrination. Certainly the crazy people are more likely to do crazy things than people who are not crazy.
The populations are too mixed these days anyway.
A few ignorant blowhards who declare people from above or below a certain latitude to be the enemy destroy any hope of success right from the start.
You are right. However, the big nation states are collapsing and smaller and smaller groups of people are able to use force effectively. Check out the book, The Sovereign Individual by Davidson and Rees-Mogg. It was written a few years ago, but it predicts the break-up of countries and a chaotic future, at least for a while.
They say new or new versions of older political organization will arise. We may see the return of city-states. It’s an interesting book.