The Battle of Antietam, also called the Battle of Sharpsburg, particularly in the Southern United States, took place during the American Civil War on September 17, 1862, between Confederate General Robert E. Lee's Army of Northern Virginia and Union Major General George B. McClellan's Army of the Potomac near Sharpsburg, Maryland, and Antietam Creek. Part of the Maryland Campaign, it was the first field army–level engagement in the Eastern Theater of the American Civil War to take place on Union soil. It remains the bloodiest day in American history, with a tally of 22,727 dead, wounded, or missing on both sides. Although the Union Army suffered heavier casualties than the Confederates, the battle was a major turning point in the Union's favor.

Robert Edward Lee was a Confederate general during the American Civil War, toward the end of which he was appointed the overall commander of the Confederate States Army. He led the Army of Northern Virginia—the Confederacy's most powerful army—from 1862 until its surrender in 1865, earning a reputation as a skilled tactician.