Again as during the former invasion, there were eleven thousand Federal troops holding Harper's Ferry. Again, as a year before, the general commanding the Army of the Potomac wanted to save them to his army by ordering them to evacuate the place and join him in the field. Again, as on the former occasion, Halleck refused to sanction this, in spite of the obvious fact that the Confederates must certainly and easily capture the whole of that force unless it should save itself by timely retreat. About this time Hooker, in disgust at the restraints to which he was subjected, asked to be relieved of the command of the Army of the Potomac, and General Meade, a very much abler man, was appointed to succeed him. Meade instantly ordered the evacuation of Harper's Ferry--the very thing that Hooker had been forbidden to order, and thus saved eleven thousand seasoned troops to the Army with which he must presently confront Lee. On the twenty-eighth of June Lee's army lay at Chambersburg, York, and Carlisle, in Pennsylvania. In that position it threatened Baltimore, Harrisburg and Philadelphia about equally. It might move upon either at will and in either case cut off Washington. The problem of the Army of the Potomac was to find out its adversary's intention and interpose itself at whatever point interposition might seem to be most necessary. In the course of this advance, Lee made one capital blunder. It was his courteous custom, in giving orders to his higher lieutenants, to leave much to their discretion, if only by way of emphasizing his confidence in them. In this case he left much to the discretion of Stuart, who had no discretion, although he had every other good quality of the soldier. Lee ordered Stuart to make certain movements if they commended themselves to his judgment, but left him in effect free to do as he pleased, assuming that he would please to do that which was discreet, bearing in mind that the cavalry are the eyes and ears of the army. It was Stuart's chief business to find out and report every movement of the Army of the Potomac, to follow its every march, to learn what it was planning to do, and at every step to find out and report to Lee the force employed in each maneuver. Stuart was an adept in this work. No man then living--probably no man who ever lived--knew better than he did how to find out the purposes of an enemy or to estimate his strength or to determine where and when that enemy meant to strike. But by virtue of his orders, Stuart was free to do what he pleased; and it pleased him to find out how nearly his cavaliers could ride into Washington, to throw shells into that city and generally to impress himself dramatically and theatrically upon the Federal administration as a terror.