The conflict's roots lay in a tension deferred since the founding: the gap between the slave-based cotton economy of the Southern states and an industrializing, urbanizing North built on wage labor. The compromises of the 1850s — the Missouri Compromise, the Kansas–Nebraska Act, the Dred Scott decision — deepened rather than defused the rift. When the Republican Abraham Lincoln, who opposed slavery's expansion, won the 1860 election, South Carolina seceded, followed by ten more Southern states forming the Confederate States of America. On 12 April 1861 Confederate guns opened fire on Fort Sumter in Charleston harbor, beginning the war.

The South had veteran generals (Robert E. Lee, Stonewall Jackson) and the advantages of fighting on the defensive. But the North's population, railroads, foundries, and navy proved decisive. The Battle of Antietam in September 1862 — at 22,000 casualties in a single day, the bloodiest day in American history — gave Lincoln the political ground to issue the Emancipation Proclamation on 1 January 1863, redefining the war as one for both Union and freedom. In July 1863 the three-day Battle of Gettysburg broke Lee's invasion of the North; that same week the fall of Vicksburg opened the Mississippi to the Union. In 1864 General William T. Sherman captured Atlanta and conducted his 'March to the Sea,' systematically destroying Southern infrastructure — an early instance of modern strategy treating the enemy's civilian economy as a legitimate war aim. On 9 April 1865 Lee surrendered at Appomattox Court House; five days later Lincoln was assassinated at Ford's Theatre.

The war's roughly 750,000 military deaths — recent estimates push the figure as high as 850,000 — exceed America's losses in all its other wars combined. The 13th Amendment, ratified in December 1865, abolished slavery throughout the United States; the 14th (1868) and 15th (1870) granted citizenship and the vote to former slaves. But Reconstruction (1865–1877) collapsed halfway: Jim Crow laws, Ku Klux Klan violence, and racial segregation persisted for another century. Some popular narratives still frame the war as one of 'states' rights,' but the consensus of modern historians is unambiguous — the secession declarations issued by the seceding states themselves name slavery as the primary cause.

The Civil War sits on the threshold of modern warfare: rifled infantry, the telegraph, railroad logistics, ironclad ships (Monitor and Merrimack), field photography (Mathew Brady, Alexander Gardner), and strategic destruction aimed at the civilian economy. Every precursor to the industrial slaughter Europe would see fifty years later in the First World War is already visible. It also stands as an example of a modern nation-state closing — but not fully resolving — its founding contradiction between proclaimed freedom and practiced slavery, in blood. That unresolved settlement still echoes through American politics in the 21st century.