The American Civil War was fought in the United States from 1861 to 1865. After a long standing controversy over slavery and state's rights, war broke out in April 1861, when Confederates attacked Fort Sumter in South Carolina, shortly after Abraham Lincoln was elected. The nationalists of the Union proclaimed loyalty to the U.S. Constitution. They faced secessionists of the Confederate States of America advocating states’ rights to perpetual slavery and its expansion in the Americas.
Among the 34 U.S. states in February 1861, seven Southern slave states individually declared their secession from the U.S. to form the Confederate States of America. The Confederacy grew to include eleven states; it claimed two more border states (Kentucky and Missouri), the Indian Territory, and the southern portions of the Union's western territories of Arizona and New Mexico, which was organized and incorporated into the Confederacy as Confederate Arizona. The Confederacy was never diplomatically recognized by the United States government, nor was it recognized by any foreign country (although Britain and France granted it belligerent status). The states that remained loyal, including the border states where slavery was legal, were known as the Union or the North.
The North and South quickly raised volunteer and conscription armies that fought mostly in the South over four years. During this time many innovations in warfare occurred, including the development and use of iron-clad ships, ultimately changing naval strategy around the world. The Union finally won the war when General Robert E. Lee surrendered to General Ulysses S. Grant at the battle of Appomattox, which triggered a series of surrenders by Confederate generals throughout the southern states. Four years of intense combat left 620,000 to 750,000 soldiers dead, a higher number than the number of American military deaths in World War I and World War II combined, and much of the South's infrastructure was destroyed. The Confederacy collapsed, slavery was abolished, and 4 million slaves were freed. The Reconstruction Era (1863–1877) overlapped and followed the war, with the process of restoring national unity, strengthening the national government, and granting civil rights to freed slaves throughout the country. The Civil War is arguably the most studied and written about episode in American history.