An executive order by Lincoln that "freed" all of the slaves in Confederate states.
The Emancipation Proclamation (1863) was an executive order by Lincoln that proclaimed the of freedom of 3.1 million slaves under Confederate control. It did not free slaves in the Union border states, because that could anger them and prompt their secession.
The Proclamation was another of Lincoln's methods of emphasizing that the Civil War was really about slavery. It ensured that all slaves captured by the Union in their campaign would be liberated. It had little direct legal effect on the South, but it eventually led to the 13th Amendment, banning slavery in the United States.