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On September 17, 1862, 75,000 Union troops under the command of George McClellan, clashed with about 40,000 Confederate troops under the command of Robert E. Lee at Sharpsburg, Maryland. The horrible battle, which was the bloodiest day in American history, became known as the Battle of Antietam because of the creek (Antietam Creek) that ran through the battle site. The landmark battle was not a military victory for either side, but rather a moral and tactical victory for the Union. Lee's exhausted Army of Northern Virginia was forced to retreat to the Virginia side of the Potomac River. General McClellan, however, failed to order the pursuit of the fleeing Confederates, which ultimately allowed them to regroup.

Despite the inconclusive nature of the battle, president Abraham Lincoln declared the battle a significant victory for the Union. Lincoln's victorious assertion was important for northern morale because of significant defeats in Virginia, and increasing criticism from "Copperheads", Democrats who favored peaceful negotiations with the South. Furthermore, the Battle of Antietam provided an opportunity for president Lincoln to free all slaves still subjugated in the South. Five days after the battle, on September 22, 1862, Lincoln issued the Emancipation Proclamation which freed all slaves in "enemy territory" as of January 1, 1863. The announcement was hailed by abolitionists (people who opposed slavery). However, it is important to note that the new law did not free slaves being held in the border states of Delaware, Maryland, Kentucky, and Missouri. Lincoln was concerned that the issuance of a universal emancipation of all slaves would persuade those states to secede from the Union and join the Confederacy.