What was the major cause of the Civil War?

The major cause of the Civil War was the conflict over slavery, specifically the disagreement between Southern states determined to preserve and expand the institution and a growing Northern movement opposed to its expansion into new territories and increasingly hostile to its existence anywhere. Slavery was woven into nearly every political, economic, social, and constitutional dispute of the antebellum era.

Economically, slavery underpinned the southern plantation system that produced the cotton, tobacco, sugar, and rice that fueled the entire Atlantic economy. By 1860 the United States produced about 75 percent of the world's cotton supply, mostly through enslaved labor on plantations. The market value of enslaved people in the Confederate states was about 3 billion dollars, more than the combined value of all the country's railroads, factories, and banks.

Politically, slavery shaped national institutions through the Three-Fifths Clause inflating Southern congressional and electoral power, the equal Senate balance between slave and free states that compromise after compromise tried to preserve, and the disproportionate Southern hold on the presidency, Speakership, and Supreme Court. Eleven of the first 16 presidents owned enslaved people.

Constitutionally, slavery raised questions about federalism, due process, and the meaning of citizenship, all of which the Dred Scott decision of March 6, 1857 unsettled. Each major political crisis turned on slavery. The Missouri Compromise of 1820 tried to balance free and slave states. The Wilmot Proviso of 1846 sought to ban slavery from territories acquired in the Mexican-American War. The Compromise of 1850 admitted California free, organized New Mexico and Utah with popular sovereignty, and strengthened the Fugitive Slave Act. The Kansas-Nebraska Act of 1854 repealed the Missouri Compromise. The Dred Scott decision held that Black people had no citizenship rights and that Congress had no power to ban slavery in the territories. John Brown's October 16 to 18, 1859 raid on Harpers Ferry sought to ignite a slave revolt. The 1860 election split between candidates with sharply different views on slavery.

The seceding states themselves were explicit. South Carolina's December 24, 1860 Declaration of Causes cited the Northern states' resistance to fugitive slave laws and election of Lincoln as fatal. Mississippi's January 9, 1861 declaration began Our position is thoroughly identified with the institution of slavery. Texas, Georgia, and other secession declarations made similar central claims. Confederate Vice President Alexander Stephens called slavery the cornerstone of the new nation in his March 21, 1861 speech.

Other factors including economic differences, tariffs, and states' rights existed but were either ancillary to slavery or framed in ways that ultimately turned on slavery. The states' right that the seceded states meant to defend was the right to maintain slavery against perceived federal interference.