Of the 34 U.S. states in February 1861, seven Southern slave states were declared by their state governments to have seceded from the country, and the 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.^[f] The states that remained loyal to the U.S. were known as the Union.^[g] The Union and the Confederacy quickly raised volunteer and conscription armies that fought mostly in the South for four years. Intense combat left between 620,000 and 750,000 soldiers dead,^[14] along with an undetermined number of civilians.^[h] The Civil War remains the deadliest military conflict in American history,^[i] and accounted for more American military deaths than all other wars combined until the Vietnam War.^[j]

The war effectively ended on April 9, 1865, when Confederate General Robert E. Lee surrendered to Union 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 its railroads. The Confederacy collapsed, slavery was abolished, and four million enslaved Black people were freed. The war-torn nation then entered the Reconstruction era in a partially successful attempt to rebuild the country and grant civil rights to freed slaves.

The war is one of the most studied and written about episodes in U.S. history, and remains the subject of cultural and historiographical debate. Of particular interest are the causes of the Civil War and the persisting myth of the Lost Cause of the Confederacy. The American Civil War was among the earliest industrial wars. Railroads, the telegraph, steamships and iron-clad ships, and mass-produced weapons were employed extensively. The mobilization of civilian factories, mines, shipyards, banks, transportation, and food supplies all foreshadowed the impact of industrialization in World War I, World War II, and subsequent conflicts. It remains the deadliest war in American history. By one estimate, the war claimed the lives of 10 percent of all Northern men 20–45 years old, and 30 percent of all Southern white men aged 18–40.^[17]