On September 4, 1862, General Robert E. Lee, hoping to shorten the war by winning a decisive victory on Northern soil, crossed the Potomac River into Maryland. Lee planned to draw the Army of the Potomac through South Mountain into Pennsylvania and fight on ground of his choosing. His plan depended on securing his supply line down the Shenandoah Valley past Harpers Ferry—then garrisoned by nearly 13,000 Federal troops. When the Federals did not withdraw, Lee decided to attack them. From his camp near Frederick, Maryland, he divided his army into five parts. Lee gambled he could take Harpers Ferry and regroup before the Federals realized what he had done. He sent three units under the command of General T. J. "Stonewall" Jackson from Frederick to Harpers Ferry. A fourth marched into Hagerstown to guard against a rumored movement of Union troops from Pennsylvania. A fifth unit formed the rear guard at Boonesboro.
General George B. McClellan organized the Army of the Potomac into three wings and marched out of Washington along a twenty-five mile front. Learning that Lee's army was divided and marching in opposite directions well to the west, McClellan began his pursuit into western Maryland on September 11. Moving faster than Lee expected, he entered Catoctin Valley on the 13th and reached the foot of South Mountain on the 14th. The Battle of South Mountain smashed Lee's plan to invade Pennsylvania but did buy him time to concentrate his scattered army. Lee assembled his army at Sharpsburg and set up a defensive position behind Antietam Creek on the 15th. The Harpers Ferry garrison surrendered that morning. This event allowed Jackson to rejoin Lee. The Battle of Antietam was fought two days later.
In response to Lee's orders, Jackson marched via Williamsport and closed on Harper's Ferry from the north and west. McLaws moved via Brownsville Pass to occupy Maryland Heights, at the Southern end of Elk Ridge. Walker moved south and west to occupy Loudoun Heights. Lee moved with Longstreet to Hagerstown and D. H. Hill was ordered to cover the supply trains near Boonsboro.
Donated to the people of the United States by John and Candace Richards of Pennsylvania.