You are thinking about it incorrectly, there will never be pitched battles, states wouldn't secede, there won't be a war. If there is a sustained conflict it will look much more like the Troubles in Northern Ireland. There will be right wing actors and paramilitaries operating in a decentralized manner. They will start attacking individuals and infrastructure. Leading to a general state of fear. In response left wing actors will form small paramilitary cells to defend/get revenge. Mostly based on a belief that the police are either unable to or intentionally not stopping the right wing actors. As things escalate states will mostly likely be required to call up national guard troops to serve in a law enforcement role. One that they will mostly be untrained for. This will lead to abuses of power most likely in the form of arbitrary detention, torture and forced confessions. Mostly likely against both sides. This will just harden resolve and increase distrust of the government. And of course there will be the rise of political actors on both sides. Ones that most likely will not have any direct control of the paramilitaries but will use their bully pulpits to direct action while maintaining the appearance of freedom of speech. In the meantime the federal government may be so paralyzed by partisanship that it is incapable of taking meaningful action.

Yeah I don't sleep well... Sorry if you all don't now either.