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.