The Maryland campaign occurred September 4–20, 1862, during the American Civil War. The campaign was Confederate General Robert E. Lee's first invasion of the Union. It was repulsed by the Army of the Potomac under Maj. Gen. George B. McClellan, who moved to intercept Lee and his Army of Northern Virginia and eventually attacked it near Sharpsburg, Maryland. The resulting Battle of Antietam was the bloodiest day of battle in American history.