The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 2: Two Years of Grim War.
Francis Trevelyan Miller, Ed.

Leaving General Stuart with ten thousand cavalry and a part of Hill's corps to prevent Hooker from pursuing, Lee crossed the Potomac early in June, 1863, concentrated his army at Hagerstown, Maryland, and prepared for a campaign in Pennsylvania, with Harrisburg as the objective. His army was organized in three corps, under the respective commands of Longstreet, Ewell, and A. P. Hill. Lee had divided his army so as to approach Harrisburg by different routes and to assess the towns along the way for large sums of money. Late in June, he was startled by the intelligence that Stuart had failed to detain Hooker, and that the Federals had crossed the Potomac and were in hot pursuit.

Lee was quick to see that his plans must be changed. He knew that to continue his march he must keep his army together to watch his pursuing antagonist, and that such a course in this hostile country would mean starvation, while the willing hands of the surrounding populace would minister to the wants of his foe. Again, if he should scatter his forces that they might secure the necessary supplies, the parts would be attacked singly and destroyed. Lee saw, therefore, that he must abandon his invasion of the North or turn upon his pursuing foe and disable him in order to continue his march. But that foe was a giant of strength and courage, more than equal to his own; and the coming together of two such forces in a mighty death-struggle meant that a great battle must be fought, a greater battle than this Western world had hitherto known.

The Army of the Potomac had again changed leaders, and George Gordon Meade was now its commander. Hooker, after a dispute with Halleck, resigned his leadership, and Meade, the strongest of the corps commanders, was appointed in his place, succeeding him on June 28th. The two great armies--Union and Confederate--were scattered over portions of Maryland and southern Pennsylvania. Both were marching northward, along almost parallel lines. The Confederates