The American Civil War (1861–1865), among other names also known as the War Between the States, was a civil war in the United States of America. Eleven Southern slave states declared their secession from the United States and formed the Confederate States of America, also known as "the Confederacy". Led by Jefferson Davis, they fought against the United States (the Union), which was supported by all the free states (where slavery had been abolished) and by five slave states that became known as the border states.

In the presidential election of 1860, the Republican Party, led by Abraham Lincoln, had campaigned against the expansion of slavery beyond the states in which it already existed. In response to the Republican victory in that election, seven states declared their secession from the Union before Lincoln took office on March 4, 1861. Both the outgoing administration of President James Buchanan and Lincoln's incoming administration rejected the legality of secession, considering it rebellion. Several other slave states rejected calls for secession at this point.

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 volunteer army from each state to recapture federal property. This led to declarations of secession by four more slave states. Both sides raised armies as the Union assumed control of the border states early in the war and established a naval blockade. In September 1862, Lincoln's Emancipation Proclamation made ending slavery in the South a war goal, and dissuaded the British from intervening.

Confederate commander Robert E. Lee won battles in the east, but in 1863 his northward advance was turned back with heavy casualties after the Battle of Gettysburg. To the west, the Union gained control of the Mississippi River after their capture of Vicksburg, Mississippi, thereby splitting the Confederacy in two. The Union was able to capitalize on its long-term advantages in men and materiel by 1864 when Ulysses S. Grant fought battles of attrition against Lee, while Union general William Tecumseh Sherman captured Atlanta, Georgia, and marched to the sea. Confederate resistance collapsed after Lee surrendered to Grant at Appomattox Court House on April 9, 1865.

The American Civil War was one of the earliest true industrial wars. Railroads, steamships, mass-produced weapons, and various other military devices were employed extensively. The practices of total war, developed by Sherman in Georgia, and of trench warfare around Petersburg foreshadowed World War I in Europe. It remains the deadliest war in American history, resulting in the deaths of 620,000 soldiers and an undetermined number of civilian casualties. Ten percent of all Northern males 20–45 years of age died, as did 30 percent of all Southern white males aged 18–40. Victory for the North meant the end of the Confederacy and of slavery in the United States, and strengthened the role of the federal government. The social, political, economic and racial issues of the war decisively shaped the reconstruction era that lasted to 1877.

American Civil War
The Battle of Gettysburg
Date April 12, 1861 – April 9, 1865 (last shot ended June, 1865) Location Southern United States Result Union victory Territorial integrity of the United States of America preserved Reconstruction Slavery abolished
Belligerents
United States of America (Union)	Confederate States of America (Confederacy)
Commanders and leaders
Abraham Lincoln Winfield Scott George B. McClellan Henry Wager Halleck Ulysses S. Grant Gideon Welles and others	Jefferson Davis P.G.T. Beauregard Joseph E. Johnston Robert E. Lee Stephen Mallory and others
Strength
2,100,000	1,064,000
Casualties and losses
110,000 killed in action 360,000 total dead 275,200 wounded	93,000 killed in action 260,000 total dead 137,000+ wounded