Slavery was certainly not the only reason for the Civil War. The causes of the war were a complex series of events, including slavery, that began long before the first shot. States rights under the Constitution, political turmoil, the definition of freedom, the preservation of the Union, the fate of slavery, and the structure of our society and economy could all be listed as significant contributing factors.
Not the least of these factors was the tariff policy: In 1832, Henry Clay introduced the "Tariff of 1832." President Andrew Jackson countered Clay's excessive tariffs with a more moderate but still high tariff bill he supported.
Jackson's tariff bill ultimately prevailed and passed both the House and Senate by a wide margin. South Carolina, however, held out. The state called a special session of the legislature to consider its options.
With the Union appearing to be breaking up and civil war seemingly looming on the horizon, Jackson ordered army and navy forces to stand ready in Virginia and Charleston. The South Carolina legislature declared Jackson's tariffs null and void and prohibited their collection within the state on November 24, 1832.
Jackson introduced what came to be known as the "Force Bill," which granted the government the authority necessary to enforce tariffs. The U.S. Army began stocking ammunition in North Carolina.
Under pressure, South Carolina worked out a compromise. Calhoun was elected to the Senate and therefore resigned as vice-president. Jackson had Clay introduce a new tariff bill that reduced tariffs over ten years but still greatly favored the industrial north over the agricultural south.
Both the Force Bill and the new tariff passed in 1833.
Here's a decent recap of the tariff and trade problems the south had with the north from Douglas Harper's website (He's a historian fron Pennsylvania).
http://www.etymonline.com/cw/economics.htm