<human>:  When did the Battle of Antietam take place?
<bot>: The Battle of Antietam took place on September 17, 1862. It was fought between the Union Army of the Potomac and the Confederate Army of Northern Virginia. The Union Army of the Potomac consisted of the Army of the Potomac, commanded by General George McClellan, and the Army of the Shenandoah, commanded by General Ambrose Burnside. The Confederate Army of Northern Virginia was led by General Robert E. Lee.