What was the primary cause of the Civil War?

The primary cause of the Civil War was the disagreement over the expansion of slavery into new western territories, a dispute that escalated steadily from 1820 through 1860 and that the seceding southern states themselves identified as their fundamental reason for leaving the Union. The expansion question, rather than the existence of slavery in states where it already operated, was the constant flashpoint.

Southern leaders increasingly believed that slavery had to expand to survive. New land and new slave states would maintain the political balance in the Senate, provide fresh agricultural opportunities for ambitious planters, give an outlet to the natural increase of enslaved populations, and prevent slavery from being surrounded by hostile free states that could ultimately legislate against it. Northern leaders increasingly believed that slavery had to be contained to die. Free Soil ideology argued that the institution degraded free labor wherever it existed and that western territories should be reserved for free white farmers. Republicans, founded in 1854, made non-extension of slavery their central platform.

Each major political crisis of the antebellum era turned on the territorial question. The Missouri Compromise of 1820 admitted Missouri as a slave state, Maine as a free state, and banned slavery north of latitude 36 degrees 30 minutes in the Louisiana Purchase. The Wilmot Proviso of August 8, 1846 sought to ban slavery from any territory acquired from Mexico but failed in the Senate. The Compromise of 1850 admitted California free and organized New Mexico and Utah territories with popular sovereignty on slavery. The Kansas-Nebraska Act of May 30, 1854 repealed the 36 degrees 30 minutes line and applied popular sovereignty to all federal territory.

Bleeding Kansas from 1854 to 1859 produced violence between proslavery and antislavery settlers. The Dred Scott decision of March 6, 1857 ruled that Congress had no power to prohibit slavery in any territory. John Brown's raid on Harpers Ferry on October 16 to 18, 1859 sought to ignite a slave revolt. Abraham Lincoln's election on November 6, 1860 came on a Republican platform pledged to prohibit slavery's extension while leaving it untouched in existing slave states.

Southern states interpreted Lincoln's election as a sign that the political balance had tipped permanently against them. Their secession declarations made the slavery cause explicit. South Carolina's Declaration of Causes of December 24, 1860 cited Northern resistance to fugitive slave laws and the election of a hostile administration. Mississippi's Declaration of January 9, 1861 stated that "Our position is thoroughly identified with the institution of slavery." Confederate Vice President Alexander Stephens declared in his March 21, 1861 Cornerstone Speech that the Confederate constitution was founded on the great truth that slavery is the natural condition of African Americans.