The Battle of Antietam, also known as the Battle of Sharpsburg, resulted in Confederate forces under General retreating from the state of Maryland during his first invasion of Union territory. Although losses on both sides were so great that they marked the single bloodiest day in United States history, Lee's forced retreat from Maryland was considered a Union victory and a turning point in the war. Seizing upon the opportunity presented by a long overdue Union victory, President Abraham Lincoln issued the Emancipation Proclamation 4 days later.
The Confederate retreat from Union territory after the Battle of Antietam put an end to the South's hopes of France or England recognizing their independence. Both countries were watching Lee's invasion of the North closely and may have intervened on behalf of the Confederacy had the invasion been a success.
General George B. McClellan, the commander of Union forces at the Battle of Antietam, was relieved of his command by President Lincoln less than 2 months later for failing to pursue General Lee's defeated troops as they retreated into Confederate territory.