The American Civil War (1861–1865) was a major war between the United States (the "Union") and eleven Southern slave states which declared that they had a right to secession and formed the Confederate States of America, led by President Jefferson Davis. The Union, led by President Abraham Lincoln and the Republican Party, opposed the expansion of slavery into territories owned by the United States and rejected any right of secession. Fighting commenced on April 12, 1861, when Confederate forces attacked a United States (federal) military installation at Fort Sumter in South Carolina.

During the first year, the Union asserted control of the border states and established a naval blockade as both sides raised large armies. In 1862 large, bloody battles began, causing massive casualties as a result of incompatibility between new weapons and old battlefield tactics. In September 1862, Lincoln's Emancipation Proclamation made the freeing of slaves in the South a war goal, despite opposition from northern Copperheads who tolerated secession and slavery. Emancipation reduced the likelihood of intervention from Britain and France on behalf of the Confederacy. In addition, the goal also allowed the Union to recruit African-Americans for reinforcements, a resource that the Confederacy did not dare exploit until it was too late. War Democrats reluctantly accepted emancipation as part of total war needed to save the Union. In the East, Confederate general Robert E. Lee assumed command of the Army of Northern Virginia and rolled up a series of victories over the Army of the Potomac, but his best general, Thomas Jonathan "Stonewall" Jackson, was killed at the Battle of Chancellorsville in May 1863. Lee's invasion of the North was repulsed at the Battle of Gettysburg in Pennsylvania in July 1863; he barely managed to escape back to Virginia. The Union Navy captured the port of New Orleans in 1862, and Ulysses S. Grant seized control of the Mississippi River by capturing Vicksburg, Mississippi in July 1863, thus splitting the Confederacy.

By 1864, long-term Union advantages in geography, manpower, industry, finance, political organization and transportation were overwhelming the Confederacy. Grant fought a number of bloody battles with Lee in Virginia in the summer of 1864. Lee's defensive tactics resulted in extremely high casualties for Grant's army, but Lee lost strategically overall as he could not replace his casualties and was forced to retreat into trenches around his capital, Richmond, Virginia. Meanwhile, William Tecumseh Sherman, the leader of the Union Military Division of the Mississippi, captured Atlanta, Georgia. Sherman's March to the Sea destroyed a hundred-mile-wide swath of Georgia. In 1865, the Confederacy collapsed after Lee surrendered to Grant at Appomattox Court House; all slaves in the Confederacy were freed by the Emancipation Proclamation. Slaves in the border states and Union controlled parts of the South were freed by state action or by the Thirteenth Amendment.