How about between Florida and New York? Or cities in red states vs their state government? It’s not zero anymore, especially when politicians challenging federal authority (eg Texas with border control) in obviously illegal ways.

We have numerous examples - Jan 6th, the Bundy standoffs, Oklahoma City, the Black Wall Street bombings - the risk isn’t zero.

That's because there's a common government that's more powerful than both of them and will stop any such war. But that common government still has to maintain its power with an army that can function.

Obviously civil wars happen all the time in places where there isn't a single powerful enough government to keep them suppressed.

Maybe you want one world government or a military alliance that includes every country and they all fight against any local wars no matter what. But what happens if some big enough group feels (or is) oppressed by that government and tries to fight it? Oppression by the majority forever is better than independence for anyone?

It's zero now. But if they ever find themselves in distinct nations then the odds will change.