From the "Since you asked…" file.

South Carolina State Senator Glenn McConnell wonders…

“Why was the war fought — was it about slavery or states’ rights?”

Since you asked, slavery, but just to make sure, let's ask Alexander Stephens, Vice President of the Confederate States Of America…

Our new Government is founded upon exactly the opposite ideas; its foundations are laid, its cornerstone rests, upon the great truth that the negro is not equal to the white man; that slavery, subordination to the superior race, is his natural and moral condition.

So, slavery it is.