to be fair the military is pretty right wing and i could see most of the us military siding with americans instead of the government or just refusing to fight
and for the civil war being supplied by foreign powers i could see the republicans being supplied by russia and perhaps china could supply the democrats but i see those two countries seeing this as an opportunity to invade neighboring countries to expand, north korea with help of china could finally take out south korea if nato just completely disbands after the civil war ignites, but i most definitely see russia invading the baltics and ukraine.
speaking mapwise the civil war would most likely look like this
https://media.discordapp.net/attachments/630933404015263755/660214985233793024/image0.jpg the democrats have a few coastlines for supplying but i can see the republicans quickly taking these out to avoid any foreign support
and for the navy i think we could say 8/10th of it would side with the republicans so i can see a heavy blockade around democratic coastlines ob both west and eastern fronts.