STATES RIGHTS
STATES RIGHTS
STATES RIGHTS
Explanation: The US Civil War is sometimes said, by a particularly vile tradition known as the "Lost Cause", to have been started over "states' rights".
Other than being patently untrue in and of itself, as the seceding Confederacy actually denied states more rights than the Union did, the simple fact is that the war was about slavery. It's the only issue of the period where "states' rights" (as states had the right to allow or forbid slavery) even could conceivably come into play.
Okay I'll be really pedantic and say that the war was about states' rights, particularly a state's right to secede. Of course they decided to secede because of slavery so it's a moot point, but the North didn't give a shit about slavery; they just wanted to keep those damn traitors in the Union.
Most of the states cited the owning slaves as their cause for succeeding in their Articles/Causes of Succession.
https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states
https://www.battlefields.org/learn/primary-sources/secession-acts-thirteen-confederate-states
I want to say 3 of them didn't mention it directly. Three.