What caused the American Civil War? Slavery and the Union

What caused the Civil War, in a word, was slavery. Eleven Southern states left the United States in 1860 and 1861 to protect slavery, which they feared the newly elected President Abraham Lincoln and his Republican Party would first stop from spreading and then destroy. The war that followed, from 1861 to 1865, kept the Union together and ended slavery. That is the consensus of historians, and it is what the seceding states said themselves.

In short

• The central dispute was whether slavery could expand into the western territories, which would decide the balance of power in Congress and slavery's future.
• The seceding states named slavery as their reason in their own official declarations.
• "States' rights" was argued for slavery's sake, and the South demanded federal power over Northern states when it served slavery.

Why was slavery at the center of American politics?

By 1860 nearly 4 million people were enslaved in the United States, almost all in the 15 slave states of the South. Their labor grew cotton, the country's biggest export, and slaveholders' human property was valued at more than $3 billion in 1860 money. The Northern states had abolished slavery, mostly gradually, between the 1770s and the early 19th century.

The country had kept an uneasy balance between free and slave states. The fight was over what came next. Every new territory in the West would become a state, with two senators. If those states were free, slavery would be outvoted in Congress and, Southern leaders feared, eventually ended. If they were slave states, slavery would grow. So the question that split the country was not mainly whether slavery would survive where it already existed, but whether it could spread.

How did the crisis build, from 1850 to 1861?

Year	Event	Why it mattered
1820	Missouri Compromise	Banned slavery in the northern Louisiana Purchase lands, above latitude 36°30′
1850	Compromise of 1850	California entered free; a harsh Fugitive Slave Act made Northerners help return people who had escaped
1852	Uncle Tom's Cabin	Harriet Beecher Stowe's novel turned many Northern readers against slavery
1854	Kansas–Nebraska Act	Let settlers vote on slavery, overturning the 1820 line; the Republican Party formed to oppose its spread
1854–1861	"Bleeding Kansas"	Pro- and anti-slavery settlers fought for control of the territory
1857	Dred Scott decision	The Supreme Court ruled that Black Americans could not be citizens and that Congress could not ban slavery in the territories
1859	John Brown's raid	An abolitionist's attempt to arm enslaved people at Harpers Ferry, Virginia, terrified the South
1860	Lincoln elected	He won without a single Southern electoral vote, on a platform of stopping slavery's expansion
1860–1861	Secession	South Carolina left on 20 December 1860; six more Deep South states followed by February 1861
1861	Fort Sumter	Confederate guns fired on the federal fort in Charleston harbor on 12 April; the war began

Lincoln did not promise to abolish slavery where it existed. He said he had no lawful power to. But he and his party called slavery a wrong and promised to stop it spreading, which Southern leaders took as a sentence of slow death for it. After Fort Sumter, Lincoln called for 75,000 volunteers to put down the rebellion, and four more states, Virginia, Arkansas, North Carolina and Tennessee, joined the Confederacy rather than fight it.

What did the seceding states say?

Four states issued formal declarations of their reasons, and they leave little doubt. Reading them is the quickest way to settle the question.

State	Date	In its own words
South Carolina	24 December 1860	It complained of "an increasing hostility on the part of the non-slaveholding States to the institution of slavery"
Mississippi	January 1861	"Our position is thoroughly identified with the institution of slavery"
Texas	2 February 1861	It held that the states' governments were "established exclusively by the white race, for themselves and their posterity"

Georgia's declaration made the same case at length. The Confederacy's vice president, Alexander Stephens, put it plainest in his Cornerstone Speech at Savannah on 21 March 1861: the new government's "foundations are laid, its corner-stone rests, upon the great truth that the negro is not equal to the white man".

Wasn't the war about states' rights?

That claim spread after the war, as part of the "Lost Cause", a story told by former Confederates in which the South fought for constitutional principle, not slavery. Historians reject it, for a reason the documents make clear: the right the seceding states cared about was the right to own slaves, and they wanted federal power used to protect it.

South Carolina's own declaration is the best evidence. It protested that 13 Northern states, from Maine to Iowa, "have enacted laws which either nullify the Acts of Congress or render useless any attempt to execute them", meaning the Fugitive Slave Act, according to the text at Yale's Avalon Project. In other words, South Carolina left the Union partly because Northern states were using their rights to refuse to return escaped people. The Dred Scott decision, which Southern leaders welcomed, likewise used federal power to stop the territories being closed to slavery.

Tariffs and economic differences were real, but they were not new in 1860 and no seceding state named them as its main grievance. The economic stakes that mattered most were the value of enslaved people and the land to expand into.

How did the war end, and what did it change?

The war lasted four years and killed about 620,000 soldiers, about 2% of the population, according to the American Battlefield Trust, with some demographic studies estimating more. Disease killed more men than battle did.

It began as a war to restore the Union and became a war to end slavery. Lincoln's Emancipation Proclamation of 1 January 1863 declared enslaved people in the rebelling states free and opened the Union army to Black soldiers. Almost 200,000 Black soldiers and sailors served, the National Archives records. The Confederacy's main army, under Robert E. Lee, surrendered at Appomattox Court House, Virginia, on 9 April 1865, and Lincoln was assassinated five days later.

The 13th Amendment abolished slavery across the country in December 1865. The news of freedom reached Galveston, Texas, on 19 June 1865, the origin of Juneteenth. The 14th Amendment of 1868 then made everyone born in the country a citizen with equal protection of the laws, reversing Dred Scott and remaking the Constitution. How far those promises were kept, through Reconstruction and the century of segregation that followed, is its own long story, and the way the war's causes were later retold is a lesson in how history gets written.

Check yourself

• Without looking back: why did the expansion of slavery into the West matter so much more than slavery where it already existed?
• What does South Carolina's complaint about Northern laws show about the "states' rights" explanation?
• Name four events from 1850 to 1861 that pushed the country toward war, and say what each one did.

Sources

• Declaration of the immediate causes which induce and justify the secession of South Carolina — Avalon Project, Yale Law School, 1860
• A declaration of the immediate causes which induce and justify the secession of the State of Mississippi — Avalon Project, Yale Law School, 1861
• A declaration of the causes which impel the State of Texas to secede from the Federal Union — Avalon Project, Yale Law School, 1861
• Cornerstone Speech — Alexander Stephens, 1861, American Battlefield Trust
• Civil War casualties — American Battlefield Trust, 2023
• The Emancipation Proclamation — U.S. National Archives