Of the 34 U.S. states in February 1861, seven Southern slave states were declared by partisans to have seceded from the country, and the so-called Confederate States of America was organized in rebellion against the U.S. constitutional government. The Confederacy grew to control at least a majority of territory in eleven states, and it claimed the additional states of Kentucky and Missouri by assertions from native secessionists fleeing Union authority. These states were given full representation in the Confederate Congress throughout the Civil War. The two remaining 'slave' states, Delaware and Maryland, were invited to join the Confederacy, but nothing substantial developed due to intervention by federal troops.

The Confederate states were never diplomatically recognized as a joint entity by the government of the United States, nor by that of any foreign country. The states that remained loyal to the U.S. were known as the Union.[f] The Union and the Confederacy quickly raised volunteer and conscription armies that fought mostly in the South over the course of four years. Intense combat left 620,000 to 750,000 people dead, more than the number of U.S. military deaths in all other wars combined.

The war effectively ended April 9, 1865, when General Robert E. Lee surrendered to General Ulysses S. Grant at the Battle of Appomattox Court House. Confederate generals throughout the southern states followed suit, the last surrender on land occurring June 23. Much of the South's infrastructure was destroyed, especially the transportation systems. The Confederacy collapsed, slavery was abolished, and four million black slaves were freed. During the Reconstruction era that followed the war, national unity was slowly restored, the national government expanded its power, and civil and political rights were granted to freed black slaves through amendments to the Constitution and federal legislation. The war is one of the most studied and written about episodes in U.S. history. Also see: https://en.wikipedia.org/wiki/American_Civil_War