On September 17, 1862, 75,000 Union troops the command of George McClellan, clashed with about 40,000 Confederate under the command of Robert E. Lee at Sharpsburg, Maryland. The horrible , which was the bloodiest day in American history, became as the Battle of Antietam of the creek (Antietam Creek) that ran the battle site. The landmark battle was not a military victory for either side, but rather a moral and tactical victory the North. Lee's exhausted Army of Northern Virginia was forced retreat to the Virginia side of the Potomac River. General McClellan, however, to order pursuit of the fleeing Confederates, ultimately allowed them to regroup.