My grandfather is a diehard conservative and refuses to believe that slavery was the primary cause for the Civil War (guess it's a passive-agressive, white pride thing). I'm submitting this question in hopes for a non-refutable, definite answer that can simply change his opinion on it. I am aware that you can lead a horse to water, but can't make him drink, but after months of him jabbing at me and my "liberal beliefs", I want to finally deliver the best argument I can in hopes that he will at least maybe consider the alternative. Thanks!
The declaration of causes is some of the best evidence for seeing that the American Civil War was about slavery. Essentitally when the states seceded Virginia, South Carolina, Mississippi, Georgia and Texas all sent out proclamations stating why they left the Union. Slavery features heavily in all of them but Mississippi is certainly the most egregious stating:
"Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. [snip] These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization."
It is difficult to deny the fact that the American Civil War was about slavery when the people involved state it as such. Here you go, and good luck http://www.civilwar.org/education/history/primarysources/declarationofcauses.html#Mississippi