Confederate General Robert E. Lee's army pulls away from Antietam Creek, near Sharpsburg, Maryland, and heads back to Virginia. The day before, at the Battle of Antietam, Lee's force had engaged in the bloodiest one-day battle of the Civil War against the army of General George B. McClellan. The armies struggled to a standstill, but the magnitude of losses forced Lee to abandon his invasion of Maryland.
The significance of the battle was not Lee's withdrawal, but McClellan's lack of pursuit. When Lee settled into a defensive line above Antietam Creek on September 16, he had only about 43,000 troops. McClellan had around 50,000 in position on September 17, with many more on the way.
On September 18, the armies remained in their positions without fighting. By this point, Lee was highly vulnerable. His army had its back to the Potomac River, just a few miles away, and a quarter of his force had been lost in the previous day's battle. And after more than two weeks of marching, his men were tired. McClellan, on the other hand, welcomed thousands of additional troops on September 18. But, although he outnumbered Lee's troops by almost three times, McClellan did not pursue Lee. In fact, despite constant urging from President Abraham Lincoln and Chief of Staff Henry Halleck, McClellan did not move toward Virginia for over a month. McClellan overestimated the size of Lee's force, assuming that Lee had nearly 100,000 troops in his command, and insisted that the fall of Harpers Ferry, Virginia, on September 15 allowed an additional 40,000 Confederate troops—in his inflated estimation—to fight at Antietam.
In McClellan's defense, it should be noted that his soldiers were extremely fatigued after the Battle of Antietam, which was the bloodiest day of the war. It would be difficult to rally them for another attack; but certainly not impossible. Instead, Lee was allowed to escape with his command intact. A chance to destroy the Confederate Army of Northern Virginia was lost, and the war lasted another two and a half years.