Sectional tensions had long existed between the states located north of the Mason–Dixon line and those south of it, primarily centered on the "peculiar institution" of slavery and the ability of states to overrule the decisions of the national government. During the 1840s and 1850s, conflicts between the two sides became progressively more violent. After the election of Abraham Lincoln in 1860 (who southerners thought would work to end slavery) states in the South seceded from the United States, beginning with South Carolina in late 1860. On April 12, 1861, forces of the South (known as the Confederate States of America or simply the Confederacy) opened fire on Fort Sumter, whose garrison was loyal to the Union.

What dividing line separated slave states from free states?