Everyone seems to have this romantic idea of a US Civil War would be like. It's either "76 million gun owners becoming patriots" or the US military will crush the secessionists with some airstrikes and drone strikes, all in time for dinner.

The first stage would be political chaos. Some US governors will see this an attempt to seize even more power and side with the Secessionists (looking at you, DeSantis, you shit head). Some US governors will wait to see if a political solution can be sought before picking sides, and other governors will side the US government. Congress will have to figure out what do with US Representatives from Secessionist States. Some members of the military will start to defect or desert for various reasons.

If a political comprise can't be sought, we move to the second stage. Secessionist States start seizing US military bases and their assets and more members of the US military and Secessionist States start to desert/defect. Russia, China, Iran, and other countries sensing an opportunity, start to exploit the ongoing chaos. This includes massive disinformation campaigns, funding violent organizations, and isolating US allies.

A small amount of far-right militias sensing an opportunity with the US government dealing with the beginnings of a civil act, start to act. Small bombings and assassinations to further their political goals. Conservatives in Northern California start terror campaigns in Southern California. Progressive groups start being targeted and band together for safety. Foreign interference becomes more involved. Refugees start fleeing.

Third stage is full out war. Battles between Secessionist forces and the US Military start happening. Every state has either decided to join one side or goes their own way. Political crises pop up in US territories. Local insurgencies break out amongst groups fighting for power as central governments are pre-occupied with fighting a civil war. Foreign inference is at a maximum with direct financial, military, or logistical support to whatever group aligns with foreign powers.

We saw this with happen with the Iraq War with it's multitude of Shiite and Sunni militias fighting each other and the US. Same thing happened in Syria, with groups supporting the government against those fighting against the government and the Kurds. We saw what can happen with a dedicated low tech insurgency can do in Afghanistan and Vietnam against a far more advanced military.

At best, the US is 11 different countries trying to be one country.. At worst, the US is 50 different countries trying to be one country.

Robert Evans of Behind the Bastards wrote an article about the beginnings of a civil war.