Let's talk about how this would actually go down. Let's assume a similar map to the original civil war, with the entire western half of the country going red aside from california, washingon, and a few of the northernmost states. The initial state to secede will likely be one of the carolinas, or alabama. Then the others will join in according to the map. The union at this point looks to be disadvantaged, however the populations are about even. Refugees fleeing to the union would change this, but not enough to make a significant difference. It would take weeks before war is formally declared, because the sitting president will probably try a peaceful resolution to begin with, before destroying american cities.
So at this point the two sides have declared war on eachother and are ready to fight with all available equipment. The south collects all weapons and ammo from military bases, and forms a militia. I'd put the size of it at about 400,000 men and women total. This number is as low as it is because I sense that most people would not be willing to pick up arms against the Union, but so that this could actually be a competition, I gave them an army. And throughout this I'll be giving the south the most favorable circumstances possible. The Union I'm going for a very low estimate and say that the army drops to 400,000 due to a desire not to fight Americans. (As I said, most favorable) So the stage is set, let the war begin.
The Union is probably going to immediately start on the offensive moving down, and the south is going to move west. California will be the battlefield that decides the war. Southern hackers have managed to disable all drones, so this battle will be with planes and men. The initial invasion southward will be met with heavy resistance as the Union struggles against the defending southern army trenched in. That front will be a war of attrition until reinforcements arrive.
In California the south meets with little to no resistance pushing in. California was never ready for an invasion of this magnitude, and the sitting president knows this. He dispaches 80 battle tanks, and 50 planes along with 50,000 men to California. He/She cannot allow California to fall. Despite flying through enemy territory, the flight encounters no real troubles and the soldiers arrive as the South finally reaches Los Angeles. The battle is heavily weighted in the Union's favor and the south is rapidly deafeted. Despite the number of bases in southern states, the Union still has superiour air and ground vehicles. Although the Union does take heavy losses, so for the rest of the war they are stationed in California.
Back to the southern assualt, the south just does not have the supplies to survive an assault from bombers for any extended period of time. Even with the number of fighter jets they have, the north still has access to every fighter plane ship in the world.
So the front is broken and the north moves through. Finally they reach Austin (because of course the capital is in Texas, we're assholes) and the south surrenders.
The north wins pretty handily even with a massive disadvantage. Although the civilian fatality rate will rival that of defeated soldiers.
What's really a problem is that the north will have to pull in all global stationed troops and vehicles, and that will allow a lot of things to happen. First North Korea will probably make a move on South Korea. Isreal will stutter but'll be fine since America isn't the only one supporting them. The middle east will probably either stabilize or fall under the control of various dictators in the resulting power vacuum.
As well we must consider that this war will destroy our economy. We'll likely default on our debt, and the damage the war caused will likely halve our GDP. America will go into a deep depression, and so will the world. If America were to fight itself, the world would devolve into chaos. I've made a lot of guesses here, but nobody really knows what would happen if the American presence and trade just dissappeared.
Well if you've made it this far, sorry about the long post. I hope it wasn't too off-topic or crazy. I just enjoy thinking through imaginary war scenarious. Anyway you've endured my shit long enough, here's a man boarding a spaceship.
8======D~~~~~ <--- that's the space exhaust.