The American Civil War (1861-1865), which is also known by several other names, was a civil war between the United States of America (the "Union") and the Southern slave states of the newly formed Confederate States of America under Jefferson Davis. The Union included all of the free states and the five slaveholding border states and was led by Abraham Lincoln and the Republican Party. Republicans opposed the expansion of slavery into territories owned by the United States, and their victory in the presidential election of 1860 resulted in seven Southern states declaring their secession from the Union even before Lincoln took office. James McPherson, Drawn With the Sword, from the article Who Freed the Slaves? The Union rejected secession, regarding it as rebellion.

Hostilities began on April 12 1861, when Confederate forces attacked a U.S. military installation at Fort Sumter in South Carolina. Lincoln responded by calling for a large volunteer army, then four more Southern states declared their secession. In the war's first year, the Union assumed control of the border states and established a naval blockade as both sides massed armies and resources. In 1862, battles such as Shiloh and Antietam caused massive casualties unprecedented in U.S. military history. In September 1862, Lincoln's Emancipation Proclamation made ending slavery in the South a war goal, which complicated the Confederacy's manpower shortages.

In the East, Confederate commander Robert E. Lee won a series of victories over Union armies, but Lee's reverse at Gettysburg in early July, 1863 proved the turning point. The capture of Vicksburg and Port Hudson by Ulysses S. Grant completed Union control of the Mississippi River. Grant fought bloody battles of attrition with Lee in 1864, forcing Lee to defend the Confederate capital at Richmond, Virginia. Union general William Sherman captured Atlanta, Georgia, and began his famous March to the Sea, devastating a hundred-mile-wide swath of Georgia. Confederate resistance collapsed after Lee surrendered to Grant at Appomattox Court House on April 9, 1865.

The war, the deadliest in American history, caused 620,000 soldier deaths and an undetermined number of civilian casualties, ended slavery in the United States, restored the Union by settling the issues of nullification and secession and strengthened the role of the Federal government. The social, political, economic and racial issues of the war continue to shape contemporary American thought.