Why did the Civil War start?

The Civil War started because eleven Southern states seceded from the United States to preserve and expand the institution of slavery, and the Lincoln administration refused to recognize secession or relinquish federal property in seceded states, so when South Carolina forces fired on Fort Sumter on April 12, 1861 the war began.

The deeper causes had been building for decades. The expansion of slavery into new western territories raised existential questions for both sides. The Missouri Compromise of 1820, the Compromise of 1850, the Kansas-Nebraska Act of 1854, the Bleeding Kansas violence of 1854 to 1859, the Dred Scott decision of 1857, and John Brown's raid on Harpers Ferry on October 16 to 18, 1859 each escalated tension.

The 1860 presidential election crystallized the crisis. Abraham Lincoln won the Republican nomination on a platform opposed to slavery's expansion (though not its abolition where it already existed) and won the November 6, 1860 election with 39.8 percent of the popular vote in a four way race against Stephen A. Douglas (Democrat), John C. Breckinridge (Southern Democrat), and John Bell (Constitutional Union). Lincoln received no electoral votes from any slave state and was not on the ballot in ten of them.

Southern political leaders interpreted his election as the triumph of an antislavery North that would inevitably constrain and eventually abolish slavery. South Carolina seceded on December 20, 1860, declaring in its formal declaration of causes that the entire purpose was to defend slavery against an incoming hostile administration. Six more Deep South states followed by February 1861: Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas. Each formal secession declaration cited slavery as the central issue.

The seceded states formed the Confederate States of America in February 1861 and adopted a constitution that explicitly protected slavery. Confederate Vice President Alexander Stephens delivered his Cornerstone Speech on March 21, 1861 declaring that the Confederate government was founded on the great truth that the negro is not equal to the white man and that slavery is his natural condition.

The Lincoln administration refused to recognize secession. Lincoln took office on March 4, 1861 and announced he would not interfere with slavery where it existed but would not allow secession or surrender federal property. Fort Sumter in Charleston Harbor became the test. After Confederate authorities demanded surrender of the fort, Confederate batteries opened fire at 4:30 a.m. on April 12, 1861. The Civil War had begun.

Four more states (Virginia, Arkansas, Tennessee, North Carolina) seceded after Lincoln called for volunteers on April 15, 1861. Some have since argued the war was about states' rights or tariffs, but the contemporary documents make clear that the states' right Southern states meant to defend was the right to maintain slavery.