Origins of the American Civil War: "Border Ruffians" from Missouri invade Kansas and force election of a pro-slavery legislature.

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, but they disagree on the North's reasons for refusing to allow the Southern states to secede. The negationist Lost Cause ideology denies that slavery was the principal cause of the secession, a view disproven by historical evidence, including 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."