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The American Civil War (1861–1865), often referred to as The Civil War in the United States, was a civil war fought over the secession of the Confederacy. In response to the election of an anti-slavery Republican as President, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ("the Confederacy"); the other 25 states supported the federal government ("the Union"). After four years of warfare, mostly within the Southern states, the Confederacy surrendered and slavery was outlawed everywhere in the nation. Issues that led to war were partially resolved in the Reconstruction Era that followed, though others remained unresolved.

In the presidential election of 1860, the Republican Party, led by Abraham Lincoln, had campaigned against expanding slavery beyond the states in which it already existed. The Republicans strongly advocated nationalism, and in their 1860 platform they denounced threats of disunion as avowals of treason. After a Republican victory, but before the new administration took office on March 4, 1861, seven cotton states declared their secession and joined to form the Confederate States of America. Both the outgoing administration of President James Buchanan and the incoming administration rejected the legality of secession, considering it rebellion. The other eight slave states rejected calls for secession at this point. No country in the world recognized the Confederacy.