Eh, the military isn’t anything like it was 170 years ago. Currently Trump and his allies control the military, and I suspect they would ally with the south. I don’t know, I feel like the country actually separating like that would be impossible anyways. During the civil war there would be people who still remembered the US being a loose collection of largely independent states. The US also didn’t have a standing army back then. Of course communication and travel are also much easier nowadays and probably state politicians feel more allegiance to the national political parties. If it did happen, for there to be any lasting fight at all, the military bases would probably have to choose their allegiance based on their location. Which feels kinda unlikely to me, idk.