The American Civil War (1861–1865) was a major war between the United States (the "Union") and eleven Southern slave states that declared their 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 and rejected any right of secession. Fighting commenced on April 12, 1861, when Confederate forces attacked a 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 new weapons and old battlefield tactics. In September 1862, Lincoln's Emancipation Proclamation made the freeing of the slaves a war goal, despite opposition from northern Copperheads who tolerated secession and slavery. Emancipation ensured that Britain and France would not intervene to help 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, Robert E. Lee rolled up a series of Confederate 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. In the West, 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 won most of the battles in a tactical sense but on the whole lost strategically, as he could not replace his casualties and was forced to retreat into trenches around his capital, Richmond, Virginia. Meanwhile, William Tecumseh Sherman 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 outside Confederate control were freed by state action or by the Thirteenth Amendment. The full restoration of the Union was the work of a highly contentious postwar era known as Reconstruction. The war produced about 970,000 casualties, including approximately 620,000 soldier deaths—two-thirds by disease. The causes of the war, the reasons for its outcome, and even the name of the war itself are subjects of lingering controversy even today. The main results of the war were the restoration and strengthening of the Union, and the end of slavery in the United States.