The next US civil war is brewing as we speak. This time around, the lines of demarcation are going to be much more murky as to the sides in this next domestic war. This time around though, the powers that be are going to rely heavily on foreign powers to fight on their side, along with financial retaliation on an epic scale, and that will go over like a lead balloon for most Americans.
It is coming, and it will only be a matter of when.
I fear you may be correct. While I don’t think the federal government would oppose secession, if it did, the ensuing conflict would be far worse than the first civil war, because CWII would be dirty war fought by paramilitary death squads and guerillas hidden among civilian populations.