The Battle of Antietam was the deadliest single-day battle in American military history. Fought on September 17, 1862, during the American Civil War, it resulted in approximately 22,000 casualties on both sides combined. The battle took place near Sharpsburg, Maryland, and was part of Confederate General Robert E. Lee's first invasion of the North. While the battle was tactically inconclusive, it provided President Abraham Lincoln with the political cover he needed to issue the Emancipation Proclamation, which shifted the war's focus toward ending slavery, transforming the conflict's moral purpose.