Judging by the number of comments, my post on slavery as the Southern states' motive for secession in 1861 has drawn a lot more interest than I expected. Unfortunately, I have less time than usual to study and respond to comments because I am about to move to the University of Pennsylvania for the fall semester. However, those interested in this issue may want to check out the official statements of reasons for secession issued by four of the eleven seceding states in 1861 - Georgia, Mississippi, South Carolina, and Texas. All four discuss slavery far more prominently than any other issue, and three of four don't mention any issues unrelated to slavery (Georgia's statement briefly mentions disputes over the tariff, but far less prominently than slavery). Mississippi's statement gives the clearest account of the centrality of slavery to the secession decision:

Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. 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. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin.