Really? What if Texas secedes again, will that cause urban warfare in Pennsylvania? I think not, the lines will be similar as the first CW. State by state.
It’ll be rural vs city - yes some places will be less so such as Texas where Austin I’m sure will be quickly taken out, but here in Ohio it’ll be the five city enclaves vs 1/2 the suburbs and everyone else. Illinois will be Chicago and e St. Louis and Springfield vs farm country. Indiana may be mainly north vs south ... Even New York has its folks who wills seek out redress from the burden of NYC and Albany.
I see cordons around the cities and channels between the cities.