I'm going to steer this slightly in a different direction if I can, and get directly back to the title of this post.
It's been discussed here quite a bit that the motivations of the South to fight a war are complex, going beyond slavery. In the minds of the combatants, was preserving slavery front and center important? I don't know, and Dr. Tilley cited a book taken from letters that indicates a wider set of motivations.
Just to take two examples, Lee and Pickett. Both graduated USMA, Lee with the second best set of marks ever. I could look it up, but I think the best went to a guy named Mason, in the same class. My Great Grandfather Morrison graduated in 1903, something like 75th. There was some other guy, MacArthur, in his class. Lee's wife Mary Custis was descended from Martha Washington, her great grand daughter, and Lee himself was a distant cousin of George. He was as American as it gets.
By the way, he did own slaves, as did his wife who had inherited her own, and they worked on the plantation he shared with her. His mother in law had ordered they be freed in 1857, on the condition that their Arlington plantation was fiscally sound, under a sort of five year plan. Lee went to court to argue, and won, that he needed the full five years to do it. (Washington Post, May 6 2011)
Pickett served the Union with distinction against the British in Puget Sound I think in 1859.
Lee was offered command of the entire Union Army by Lincoln. He chose instead to return to Virginia.