As the title suggests, how can we make the American Civil War last much longer? By much longer, I mean at least a decade longer than OTL, although I'd like to see if we can push it for several decades. You can have it as an on-and-off conflict.
Bonus if you can go on long enough to end within a few years of 1900, extra if you can do it without any sort of breaks/pauses(ceasefires, temporary peace treaties, etc).
The only condition is that it still results in a final Union victory, otherwise let's see how long this can be stretched.