 Slavery was officially abolished by the Thirteenth Amendment which took effect on December 18, 1865.Slavery had been theoretically abolished by President Abraham Lincoln's Emancipation Proclamation which proclaimed, in 1863, that only slaves located in territories that were in rebellion from the United States were free.Since the U.S. government was not in effective control of many of these territories until later in the war, many of these slaves proclaimed to be free by the Emancipation Proclamation were still held in servitude until those areas came back under Union control.