The American Civil War (also known by other names) was a civil war fought in the United States from 1861 to 1865, between the North (Union) and the South (Confederacy). The most studied and written about episode in U.S. history, the Civil War began primarily as a result of the long-standing controversy over the enslavement of black people. War broke out in April 1861 when secessionist forces attacked Fort Sumter in South Carolina shortly after Abraham Lincoln had been inaugurated as the President of the United States. The loyalists of the Union in the North, which also included some geographically western and southern states, proclaimed support for the Constitution. They faced secessionists of the Confederate States in the South, who advocated for states' rights to uphold slavery.
Of the 34 U.S. states in February 1861, seven Southern slave states were declared by partisans to have seceded from the country, and a 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, but without territory or population therein; 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 was never diplomatically recognized by the government of the United States or by that of any foreign country. The states that remained loyal to the U.S. were known as the Union. 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.