For more than a week Lee had thus been laying waste a most rich and fertile section of Pennsylvania at his leisure. Practically, indeed, the whole State was in his grasp. Would Harrisburg or Philadelphia be the first fruits of his audacity? The prize was indeed tempting, the way open. The only real impediment was the Army of the Potomac, and Lee, too, was now anxiously asking himself what had become of that army.[19] He had foreseen that it must follow him up; that every effort would be bent to compass his destruction; and it was a foregone conclusion that he must fight somewhere, if there was either enterprise or courage left on the Union side. He had even calculated on drawing the Union army so far away from fortified places that its defeat would ensure the fall of Baltimore and Washington. But as regarding its whereabouts at the present moment, Lee was completely in the dark. In an evil hour he had allowed the bulk of his cavalry to run off on a wild-goose chase around the rear of the Federal army, so that now, in his hour of need, though without his knowing it, the whole Federal army interposed to prevent its return.[20] It is quite true that up to this time Stuart, who led this cavalry, had given so many signal proofs of his dexterity that Lee was perhaps justified in inferring that if he heard nothing from Stuart, it was because the Union army was still in Virginia. And in that belief he was acting. [Sidenote: Among Spies.] Moreover, instead of being among a population eager to give him every scrap of information, Lee was now among one where every man, woman, and child was a spy on his own movements. In the absence, then, of definite knowledge touching the Union army, he decided to march on Harrisburg with his whole force, and issued orders accordingly. [Sidenote: Union Army crosses the Potomac.] When there was no longer a shadow of doubt that Lee's whole army was on the march up the Cumberland Valley, sweeping that valley clean as it went, the Union army also crossed the Potomac, on the 25th and 26th of June, and at once began moving up east of South Mountain, so as to discharge the double duty laid upon it all along of keeping between the enemy and Washington, while at the same time feeling for him through the gaps of South Mountain as it marched. For this task the Union general kept his cavalry well in hand, instead of letting it roam about at will in quest of adventures. [Sidenote: Hooker's Plan.]