Origins of the American Civil War

The origins of the American Civil War were rooted in the desire of the Southern states to preserve and expand the institution of slavery. Historians in the 21st century overwhelmingly agree on the centrality of slavery in the conflict. They disagree on which aspects (ideological, economic, political, or social) were most important, and on the North's reasons for refusing to allow the Southern states to secede. The pseudo-historical Lost Cause ideology denies that slavery was the principal cause of the secession, a view disproven by historical evidence, notably some of the seceding states' own secession documents. After leaving the Union, Mississippi issued a declaration stating, "Our position is thoroughly identified with the institution of slavery—the greatest material interest of the world."

Background factors in the run up to the Civil War were partisan politics, abolitionism, nullification versus secession, Southern and Northern nationalism, expansionism, economics, and modernization in the antebellum period. As a panel of historians emphasized in 2011, "while slavery and its various and multifaceted discontents were the primary cause of disunion, it was disunion itself that sparked the war."

Abraham Lincoln won the 1860 presidential election. His victory triggered declarations of secession by seven slave states of the Deep South, all of whose riverfront or coastal economies were based on cotton that was cultivated by slave labor. They formed the Confederate States of America after Lincoln was elected in November 1860 but before he took office in March 1861. Nationalists in the North and "Unionists" in the South refused to accept the declarations of secession. No foreign government ever recognized the Confederacy. The refusal of the U.S. government, under President James Buchanan, to relinquish its forts that were in territory claimed by the Confederacy, proved to be a major turning point leading to war. The war itself began on April 12, 1861, when Confederate forces bombarded the Union's Fort Sumter, in the harbor of Charleston, South Carolina.