Lee’s reasons for retiring toward the Potomac were unconnected with the morale of his army.  “The difficulty of procuring supplies,” he says, “rendered it impossible to continue longer where we were.”  What he especially needed was ammunition, his supply of which had been nearly exhausted by the three days’ fighting, and it was impossible to count upon new supplies of these essential stores now that the enemy were in a condition to interrupt his communications in the direction of Harper’s Ferry and Williamsport.  The danger to which the army was thus exposed was soon shown not to have been overrated.  General Meade promptly sent a force to occupy Harper’s Ferry, and a body of his cavalry, hastening across the South Mountain, reached the Potomac near Falling Waters, where they destroyed a pontoon bridge laid there for the passage of the Southern army.

Lee accordingly resolved to retire, and, after remaining in line of battle on Seminary Ridge throughout the evening and night of the 3d and the whole of the 4th, during which time he was busy burying his dead, began to withdraw, by the Fairfield and Chambersburg roads, on the night of this latter day.  The movement was deliberate, and without marks of haste, the rear-guard not leaving the vicinity of Gettysburg until the morning of the 5th.  Those who looked upon the Southern army at this time can testify that the spirit of the troops was unsubdued.  They had been severely checked, but there every thing had ended.  Weary, covered with dust, with wounds whose bandages were soaked in blood, the men tramped on in excellent spirits, and were plainly ready to take position at the first word from Lee, and meet any attack of the enemy with a nerve as perfect as when they had advanced.

For the reasons stated by himself, General Meade did not attack.  He had secured substantial victory by awaiting Lee’s assault on strong ground, and was unwilling now to risk a disaster, such as he had inflicted, by attacking Lee in position.  The enthusiasm of the authorities at Washington was not shared by the cool commander of the Federal army.  He perfectly well understood the real strength and condition of his adversary, and seems never to have had any intention of striking at him unless a change of circumstances gave him some better prospect of success than he could see at that time.

The retrograde movement of the Southern army now began, Lee’s trains retiring by way of Chambersburg, and his infantry over the Fairfield road, in the direction of Hagerstown.  General Meade at first moved directly on the track of his enemy.  The design of a “stern chase” was, however, speedily abandoned by the Federal commander, who changed the direction of his march and moved southward toward Frederick.  When near that point he crossed the South Mountain, went toward Sharpsburg, and on the 12th of July found himself in front of the Southern army near Williamsport, where Lee had formed line of battle to receive his adversary’s attack.