The Battle of Antietam was important because it stemmed the Confederate Army's advance into the northern territories, and provided an opportunity for Lincoln to deliver the Emancipation Proclamation. Though the battle was just the first to be fought in the northern colonies, Lincoln used the retreat of the southern forces at this battle as a sign the Union had the upper hand.
The Battle of Antietam included the bloodiest day of the Civil War; nearly 23,000 soldiers were killed on Sept. 18, 1862, including over 10,000 Confederate troops and over 12,000 Union soldiers. The Union possessed superior forces, so despite its losses, it eventually drove the Confederate troops back. Although neither side landed a crushing blow in this battle, President Lincoln declared it a victory for the north.
Before the Battle of Antietam, the Confederate Army had reeled a string of crushing blows to the Union, pushing battles further north. As the two forces met near the Antietam Creek in Sharpsburg, Md., the Union seemed in peril of succumbing to General Lee's forces. Lincoln needed a victory to justify delivering his rousing Emancipation Proclamation, which he hoped would improve sentiment for the Union cause. He used the weak success of the Battle of Antietam as a form of political cover to justify his speech.