The liberals here are making two big errors: They're saying (1) that the South seceded over slavery and (2) that therefore the Civil War was fought over slavery.
But secession and the war were two very different events. The Confederacy tried to establish peaceful, normal relations with the federal government. The Confederacy offered to pay the South's share of the national debt, offered to pay compensation for federal installations in the South, and sought to make the U.S. a most-favored-nation trading partner. But the Radical Republicans and certain Northern business interests refused peaceful coexistence and pressured Lincoln into opting for coercion and war.
Seven of the 11 CSA states--the seven Deep South states--seceded mainly over slavery, but the tariff and long-standing differences over the role and scope of the federal government were also major reasons for their secession,
and they said so in no uncertain terms.
Moreover, four of the 11 Confederate states did not think slavery concerns or the tariff justified secession. Those four states--the Upper South states of AR, NC, TN, and VA--seceded over federal coercion, i.e., Lincoln's use of force to maintain the Union. The Upper South states believed the Deep South states had valid complaints about slavery and the tariff, but they did not view those complaints as justifying secession. Go read the Upper South states' secession ordinances and declarations--the only reason they cited was coercion/force (slavery complaints were only mentioned in ancillary documents and were not even mentioned in these states' ordinances and declarations). These four states initially voted
against secession, and they only changed the minds after Lincoln made it clear that he was going to use the bloodless attack that he provoked at Fort Sumter as his pretext to launch an invasion of the South.