Roughly eight miles. McClellan’s men began their pursuit on the morning of September 15 and got into position late that afternoon. They found, however, that the Army of Northern Virginia had turned and established a strong defensive position in and around the town of Sharpsburg.

Lee’s men had Antietam Creek to their front and the town in their midst. They had the Potomac River two miles to their back, making retreat unlikely if not impossible. Had McClellan known that Lee had only 30,000 men, he might have ordered an all-out assault, but the Confederate positions—and the placement of their artillery—suggested a much larger number. McClellan paused, therefore, and waited not only that evening but all of the next day. The Battle of Antietam—or Sharpsburg as the Confederates called it—had to wait until Monday, September 17.