It wasn't actually about slavery. It was about the right of states to secede from the union. Unfortunately, the people who first decided to secede were slave owners, and this stained the idea of cession. I'm an American history student from Texas, the civil war is of great interest to me. In fact, one of General Lee's aide de camp's (can't remember which one), was a South Carolinian Calvinist who said that all the slaves should be freed in order to make the CSA a more humanitarian place. Nothing came of it though, mostly because then-president Jackson and CinC Robert E. Lee were both morons and bigots.
Edit: Sorry if I was a bit snappy, but I'm sick of hearing it was about slavery, especially from people who don't even live in the U.S.!