The American Civil War (1861–1865), also known as the War Between the States (among other names), was a civil war in the United States of America. Eleven Southern slave states declared their secession from the United States and formed the Confederate States of America, also known as "the Confederacy". Led by Jefferson Davis, the Confederacy fought for its independence from the United States. The U.S. federal government was supported by twenty mostly-Northern free states in which slavery already had been abolished, and by five slave states that became known as the border states. These twenty-five states, referred to as the Union, had a much larger base of population and industry than the South. After four years of bloody, devastating warfare (mostly within the Southern states), the Confederacy surrendered and slavery was outlawed everywhere in the nation. The restoration of the Union, and the Reconstruction Era that followed, dealt with issues that remained unresolved for generations.
In the presidential election of 1860, the Republican Party, led by Abraham Lincoln, had campaigned against the expansion of slavery beyond the states in which it already existed. The Republicans were strong advocates of nationalism and in their 1860 platform explicitly 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 together 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.
