Enemies of the United States chose to attack it and suffered the consequences.
Again Opinions vary
Issues regarding the regional imbalance in tariffs stoked the embers of conflict dating back to Whig leader Henry Clay's tariff increase in 1824 and the "Tariff of Abominations" in 1828. The successor party to the Whig's, the Republican Party, pushed a huge increase (250% on some items) in the Tariff, (the Morrill Tariff), in early 1861. The shelling of the Revenue Post, Fort Sumter, in Charleston Harbor came when negotiations to end the standoff in the harbor failed.
Slavery was a sideshow by comparison, as stated by A. Lincoln to Horace Greeley in 1862.