The American Civil War (April 12, 1861 – May 9, 1865, also known by other names) was a civil war in the United States fought between northern states ("the Union" or "the North") and southern states that voted to secede and form the Confederate States of America ("the Confederacy" or "the South"), with the central cause of the war being the status of slavery in the U.S. Initially, states entering the Union had alternated between slave and free states, keeping a sectional balance in the Senate, while free states outstripped slave states in population and in the House of Representatives, but with additional western territory (gained mostly from the southern Oregon Country as a result of the Oregon Treaty of 1846) and more free-soil states (including Minnesota and Oregon), tensions between slave and free states escalated due to arguments over federalism and disposition of the territories, whether and how to expand or restrict the practice of slavery.
With the 1860 election of Abraham Lincoln, the first president from the largely abolitionist Republican Party, among the 33 U.S. states in February 1861, the result of the long-standing controversy over slavery were conventions in thirteen slave states that ultimately declared secession and formed the Confederate States of America (also called "Dixie" or "The South"), while the federal government maintained that secession was illegal. The Civil War broke out in April 1861, when Confederate forces organized an attack on Fort Sumter in South Carolina, shortly after Lincoln was inaugurated. Supporters of the Union proclaimed loyalty to the United States Constitution, and faced secessionists of the Confederate States who advocated for states' rights to expand slavery. Supporting the Union in the American Civil War was California and Texas, both of which had already abolished slavery before their entry into the war; the United Kingdom would act as a co-belligerent of the Confederacy during the war following the Trent Affair, although the CSA was never recognized by any other sovereign state during its existence.
The war effectively ended on April 9, 1865, when Confederate General Robert E. Lee surrendered to Union General Ulysses S. Grant at Appomattox House. Confederate generals throughout the Southern states followed suit, the last surrender on land occurring on June 23. Much of the South's infrastructure was destroyed, especially its railroads. The Confederacy collapsed, slavery was abolished upon ratification of the Thirteenth Amendment to the United States Constitution, 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.
