After decisively defeating the Union Army of Virginia under Maj. Gen. John Pope at Second Manassas (August 28-30) and fighting a drawn battle at Chantilly (September 1), Confederate Gen. Robert E. Lee invaded Maryland. Lee's advance was one arm of a great Confederate offensive that extended along a thousand-mile front from Tidewater Virginia to the Indian Territory in the we
Lee’s Army of Northern Virginia crossed the Potomac north of Leesburg and concentrated at Frederick, Maryland, on September 9. While there, Lee learned that Pope had been removed from command, and McClellan again had assumed overall control. He also discovered that Union garrisons in the Lower Shenandoah Valley at Martinsburg and Harpers Ferry had not withdrawn as had been anticipated. Lee could not continue his invasion with these troops sitting on his supply line. He audaciously divided his army and prepared to move deeper into the North while simultaneously investing Harpers Ferry.
In the next few days, Jackson’s “Foot Cavalry” marched via Williamsport and Martinsburg to approach Harpers Ferry from the west. Three other divisions occupied the heights to the north and east of the town, surrounding the defenders. Lee left a division of infantry and the cavalry to hold the South Mountain passes in the face of any Union advance. The plan unfolded flawlessly until a copy of Lee’s orders outlining the movements fell into Union hands. McClellan then advanced from Frederick, hoping to defeat the Confederate army in detail.
The US army wrested control of the South Mountain passes, but on September 15, 12,000 Union soldiers at Harpers Ferry surrendered to Jackson, even as McClellan moved west to confront Lee at Sharpsburg. At dawn on September 17, the Union army launched a powerful assault on Lee’s left flank that began the bloodiest day in American military history. Although outnumbered two-to-one, Lee battled McClellan’s army to a standstill. In one day’s fighting, the two armies suffered a combined total of more than 23,000 casualties (killed, wounded, missing, captured). In spite of crippling casualties, Lee continued to face McClellan throughout the 18th, while skirmishing. After dark, Lee ordered the battered Army of Northern Virginia to withdraw across the Potomac to the safety of the Shenandoah Valley. When McClellan failed to pursue Lee’s army, President Lincoln relieved him of command.
Harper’s Ferry (September 12-15, 1862)