The Battle of Antietam marked the start of a new era in American military history during the Civil War. Fought on September 17, 1862, it became the bloodiest single-day battle in American history, resulting in approximately 22,000 casualties. Antietam had significant implications, as it was a pivotal Union victory that halted Robert E. Lee's invasion of the North. The battle also provided President Abraham Lincoln the political leverage needed to issue the Emancipation Proclamation, which reframed the war's purpose toward ending slavery. This shift galvanized Northern support for the war and set a moral tone for the Union's cause.