After burning Chambersburg McCausland retreated, pursued by our cavalry, towards Cumberland.

They were met and defeated by General Kelley and driven into Virginia.
The Shenandoah Valley was very important to the Confederates, because it was the principal storehouse they now had for feeding their armies about Richmond.

It was well known that they would make a desperate struggle to maintain it.

It had been the source of a great deal of trouble to us heretofore to guard that outlet to the north, partly because of the incompetency of some of the commanders, but chiefly because of interference from Washington.
It seemed to be the policy of General Halleck and Secretary Stanton to keep any force sent there, in pursuit of the invading army, moving right and left so as to keep between the enemy and our capital; and, generally speaking, they pursued this policy until all knowledge of the whereabouts of the enemy was lost.

They were left, therefore, free to supply themselves with horses, beef cattle, and such provisions as they could carry away from Western Maryland and Pennsylvania.