Final answer:
Lincoln issued the Emancipation Proclamation following the Battle of Antietam as a strategic move to weaken the Confederacy by freeing the slaves, which not only burdened Southern resources but also altered the goals of the Civil War.
Step-by-step explanation:
President Abraham Lincoln waited for a significant Union victory before issuing the Emancipation Proclamation, and the Battle of Antietam provided such an opportunity. The preliminary Emancipation Proclamation was announced on September 22, 1862, following the battle which wasn't necessarily a decisive victory, but it was sufficient for Lincoln's purposes. The proclamation declared that slaves in the Confederate states still in rebellion as of January 1, 1863 would be 'henceforth and forever free'. Lincoln considered this a military necessity essential for the preservation of the Union. Emancipation also meant the depletion of the Confederate workforce, discouraging British recognition of the Confederacy, and allowing for the recruitment of African American soldiers into the Union army. The Proclamation changed the nature of the Civil War, making the abolition of slavery an explicit war aim, beyond just reunification.