Excerpts from Books and Wikipedia
"In July 1862, The Congress in Washington passed an act automatically freeing the slaves of anyone supporting the Southern 'rebellion.'… President Lincoln took his time, but a few days after the Confederacies failure at Antietam, he issued a preliminary Emancipation Proclamation, which stated that unless the Confederate states abandoned their rebellion, on the first day of 1863, he would declare that 'all persons held as slaves' in Confederate territory 'are and henceforward shall be free.'" [Furtado: 1001 Days]