The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States between the Union ("the North") and the Confederacy ("the South"), which was formed in 1861 by states that had seceded from the Union to preserve African American slavery, which they saw as threatened because of the election of Abraham Lincoln and the growing abolitionist movement in the North.

Decades of controversy over slavery came to a head when Abraham Lincoln, who opposed slavery's expansion, won the 1860 presidential election. Seven Southern slave states responded to Lincoln's victory by seceding from the United States and forming the Confederacy. The Confederacy seized US forts and other federal assets within its borders. The war began on April 12, 1861, when the Confederacy bombarded Fort Sumter in South Carolina. A wave of enthusiasm for war swept over the North and South, as military recruitment soared. Four more Southern states seceded after the war began and, led by its president, Jefferson Davis, the Confederacy asserted control over a third of the US population in eleven states. Four years of intense combat, mostly in the South, ensued.

During 1861–1862 in the western theater, the Union made permanent gains—though in the eastern theater the conflict was inconclusive. The abolition of slavery became a Union war goal on January 1, 1863, when Lincoln issued the Emancipation Proclamation, which declared all slaves in rebel states to be free, applying to more than 3.5 million of the 4 million enslaved people in the country. To the west, the Union first destroyed the Confederacy's river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River, while Confederate general Robert E. Lee's incursion north failed at the Battle of Gettysburg. General Ulysses S. Grant's western successes led to Lincoln's giving him command of all Union armies in 1864.

Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union general William Tecumseh Sherman, followed by his March to the Sea, which culminated in his taking Savannah. The last significant battles raged around the ten-month Siege of Petersburg, gateway to the Confederate capital of Richmond. The Confederates abandoned Richmond, and on April 9, 1865, Lee surrendered to Grant following the Battle of Appomattox Court House, setting in motion the end of the war. Lincoln lived to see this victory but was shot by an assassin on April 14, dying the next day.

By the end of the war, much of the South's infrastructure had been destroyed. The Confederacy collapsed, slavery was abolished, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in an attempt to rebuild the country, bring the former Confederate states back into the United States, and grant civil rights to freed slaves. The war is one of the most extensively studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of continuing interest is the myth of the Lost Cause of the Confederacy. The war was among the first to use industrial warfare. Railroads, the electrical telegraph, steamships, the ironclad warship, and mass-produced weapons were widely used. The war left an estimated 700,000 soldiers dead, along with an undetermined number of civilian casualties, making it the deadliest in American history. The technology and brutality of the Civil War foreshadowed the coming world wars.

The origins of the war were rooted in the desire of the Southern states to preserve the institution of slavery. Historians in the 21st century overwhelmingly agree on the centrality of slavery in the conflict—at least for the Southern states. They disagree on which aspects (ideological, economic, political, or social) were most important, and on the North's reasons for refusing to allow the Southern states to secede. The pseudo-historical Lost Cause ideology denies that slavery was the principal cause of the secession, a view disproven by historical evidence, notably some of the seceding states' own secession documents. After leaving the Union, Mississippi issued a declaration stating, "Our position is thoroughly identified with the institution of slavery—the greatest material interest of the world."

The principal political battle leading to Southern secession was over whether slavery would expand into the Western territories destined to become states. Initially Congress had admitted new states into the Union in pairs, one slave and one free. This had kept a sectional balance in the Senate but not in the House of Representatives, as free states outstripped slave states in numbers of eligible voters. Thus, at mid-19th century, the free-versus-slave status of the new territories was a critical issue, both for the North, where anti-slavery sentiment had grown, and for the South, where the fear of slavery's abolition had grown. Another factor leading to secession and the formation of the Confederacy was the development of white Southern nationalism in the preceding decades. The primary reason for the North to reject secession was to preserve the Union, a cause based on American nationalism.

Background factors in the run up to the Civil War were partisan politics, abolitionism, nullification versus secession, Southern and Northern nationalism, expansionism, economics, and modernization in the antebellum period. As a panel of historians emphasized in 2011, "while slavery and its various and multifaceted discontents were the primary cause of disunion, it was disunion itself that sparked the war."