>>18092397
This is actually one of the more interesting aspects of American Civil War alternative histories that gets completely glossed over in discussions. Just because the Union and Confederate governments made peace doesn't mean that the killing would stop. You would have a whole lot of people on both sides, Abolitionists, Southern Unionists, Virginians angry at the loss of the Northern and Western parts of their state that would have almost certainly resulted from any peace deal, Kentuckians, Missourians, and Marylanders angry at being forced to remain in the Union, who wouldn't accept the final outcome and fight on. Either the Mason-Dixon Line would have to become a demilitarized zone comparable to the two Koreas or you would have militias crossing the borders and killing each other in low-level turf wars to force a particular county or town back into the Union or into the Confederacy.
You could easily get a situation where even if the Confederacy abolishes slavery piecemeal in the 1880s (presumably to negate the risk of a US-backed slave revolt plunging the country into its own civil war), but they're still having to periodically deploy the army to put down Unionist uprisings into the 20th Century.