the emancipation proclamation was an order by u.s. president abraham lincoln to free slaves in 10 states. it applied to slaves in the states still in rebellion in 1863 during the american civil war. it did not actually immediately free all slaves in those states, because those areas were still controlled by the confederacy, or those slaves in the slave states that fought for the union, as lincoln did not have authority to free slaves in those states. it did, however, free at least 20,000 slaves immediately, and nearly all 4 million slaves (according to the 1860 united states census) as the union army advanced into confederate states. until the thirteenth amendment to the united states constitution in 1865, only the states had power to end slavery within their own borders. thus, lincoln issued the proclamation as a war measure in his role as commander-in-chief.