The American Civil War was contested in the United States between the Government (states that committed loyalty to the national accord, or 'the North') and the Confederacy (states that elected to secede, or 'the South'). The major reason for the conflict was the position of enslavement, particularly its extension into areas obtained as a consequence of the Louisiana Purchase and the Mexican–American War. On the brink of the Civil War in 1860, 4 million of the 32 million Americans (13%) were owned by African people, practically all of whom lived in the South.

Slavery was a major political problem in the United States during the nineteenth century. The Civil War was the culmination of decades of political upheaval over slavery. After Abraham Lincoln secured the 1860 United States presidential campaign on an anti-slavery extension campaign, the country fell apart. The Confederacy was formed when 7 slavery in the south republics announced their independence from the nation. Confederate armies captured government castles within their declared jurisdiction.

Crittenden Compromise attempted to prevent confrontation at the last minute, but was unsuccessful; neither parties armed for battle. Fighting erupted in April 1861, little over a month after Abraham Lincoln's first installation, when the Confederate army launched the Battle of Fort Sumter in South Carolina. The Confederacy gained the support of at minimum a plurality of land in 11 states (of the 34 U.S. states in February 1861) and claimed two others. Both sides amassed sizable volunteer and conscripted forces. Multiple years of heavy fighting occurred, primarily in the South.

The Civil War is among the most composed and researched events in American history. It is still the topic of historical and cultural dispute. The myth of the Confederacy's Lost Cause is of special importance. The American Civil War was one of the first to employ economic combat. Railways, telegraph lines, steamboats, ironclad warships, and mass-produced weaponry were widely used. The conflict killed approximately 620,000 to 750,000 troops, as well as an unspecified number of civilians. Just 5 days after Lee's capitulation, President Lincoln was killed. The Civil War is still the most lethal military battle in American history. The Civil War's science and ferocity foretold the impending World Wars.