Economically, slavery was at the heart of American expansion during the first half of the nineteenth century. After Eli Whitney developed the cotton gin, plantation slavery boomed. Southern planters and would-be planters looked westward for new lands, and the internal slave trade from the Upper South supplied hundreds of thousands of slaves to fertile cotton lands in Alabama, Mississippi, Louisiana and Texas. Despite a number of financial panics that caused prices to plunge in the short term, the overall trend for cotton prices was a rapid increase, as was the price of the slaves that worked to harvest the crop. By the time of the Civil War, cotton made up the vast majority of total US exports. All of this was based on chattel slavery.
In terms of society, slavery at first gave whites an opportunity to invest capital in a form of labor that could not only bring them riches, but could also allow them some degree of social respect. Slavery was the key to white democracy in the South, at least until the late midcentury, when rising prices meant that a decreasing percentage of white society had access to slaves. Indeed, by the time the war broke out, some southerners were beginning to argue that slavery depressed wages and limited economic opportunities for whites.
Politically, the dispute over the expansion of slavery into the western territories, which was essentially about the degree to which white slaveholding southerners could use their power to ensure protection for the institution, tore the Union apart. Rising abolitionist (or anti-slavery, which was not the same thing) sentiment in the North caused many Northern politicians to view Southern actions as increasingly less tolerable. As almost every one of the secession ordinances for the Lower South states said explicitly, they left the Union to protect the institution of slavery, which they saw as being under siege from the North.