This isn’t a terribly comforting example, as the former states of the USSR have been fighting a slow-motion civil war since 1991, of which the Russian invasion

This isn’t a terribly comforting example, as the former states of the USSR have been fighting a slow-motion civil war since 1991, of which the Russian invasion of Ukraine is simply the latest chapter. See also Chechnya, Georgia, Nagorno-Karabakh, etc. I suspect a serious political divorce in the US, and the bad blood engendered, would inevitably lead to a similar outcome.