After the second battle of Bull Run, Gen. Robert E. Lee crossed the Potomac to invade Maryland and Pennsylvania. At Frederick, Md., he divided (Sept. 10) his army, sending Stonewall Jackson to capture the large Union garrison at Harpers Ferry and thus clear his communications through the Shenandoah valley. With the remainder, Lee marched NW toward Hagerstown. Gen. George B. McClellan learned of this division of forces and moved to attack. In the battle on South Mt. (the Blue Ridge N of the Potomac, 12 mi/19 km W of Frederick) on Sept. 14, 1862, McClellan defeated Lee’s rear guard and took the passes of that range.
Lee then fell back to Sharpsburg (c.9 mi/14.5 km W of South Mt.), where his position lay behind Antietam Creek. On Sept. 15 the Harpers Ferry garrison capitulated to Jackson, who, with part of his command, joined Lee before McClellan attacked. The battle of Antietam (or Sharpsburg) opened on the morning of Sept. 17. Early assaults on Lee’s left were bloody but indecisive, and McClellan failed to press the slight Union advantage with his available reserves.
In the afternoon, Burnside’s corps crossed the Antietam over the bridge on Lee’s right and drove the Confederates back, but A. P. Hill’s division arrived from Harpers Ferry and repulsed the attack. The battle was not renewed. On Sept. 18–19, Lee recrossed the Potomac into Virginia unhindered. The fighting at Antietam was so fierce that Sept. 17, 1862, is said to have been the bloodiest single day of the war with some 23,000 dead and wounded, evenly divided between the sides.
It was a Union victory only in the sense that Lee’s invasion was stopped. McClellan has been blamed for not pursuing Lee with his superior forces. The scene of the battle of Antietam has been set aside as a national battlefield site (est. 1890). The battle influenced Lincoln’s decisions to remove McClellan and to deliver a preliminary Emancipation Proclamation.