The Battle of Antietam was the single bloodiest day of combat in American history. It was also one of the key moments in the American Civil War. For both of those reasons, Antietam is held up as one of the signature battles in American history. In reality, two large armies slammed into each other, with neither being able to gain any real ground on the other. Ultimately, it would be a Union victory because Robert E. Lee decided to command his Confederate Army of Northern Virginia to retreat at the end of the day. Union General George B. McClellan and the Army of the Potomac could then claim victory. Yet both sides lost astonishing numbers and McClellan's refusal to pursue Lee made Abraham Lincoln remove him from command for a second time.