I wasnt really sure on how to phrase it, but how would you get an American Civil War but instead of secession and break away states, have it so the sides, being 2 or more, are claiming the legacy or place of the true United States, basically how North and South Korea both claim to be the only Korea.
The POD can be any time after 1845, and you must have at least 2 factions claiming to be the United States, the less break away states the better, but the occurrence of a civil war it would most likely make some states break off in itself.