Even after such a loss of opportunity, there still remained a precious balance of advantage in McClellan’s hands. Because of its smaller total numbers, the Confederate army was disproportionately weakened by the losses in battle. The Potomac River was almost immediately behind it, and had McClellan renewed his attack on the morning of the eighteenth, as several of his best officers advised, a decisive victory was yet within his grasp. But with his usual hesitation, notwithstanding the arrival of two divisions of reinforcements, he waited all day to make up his mind. He indeed gave orders to renew the attack at daylight on the nineteenth, but before that time the enemy had retreated across the Potomac, and McClellan telegraphed, apparently with great satisfaction, that Maryland was free and Pennsylvania safe.
The President watched the progress of this campaign with an eagerness born of the lively hope that it might end the war. He sent several telegrams to the startled Pennsylvania authorities to assure them that Philadelphia and Harrisburg were in no danger. He ordered a reinforcement of twenty-one thousand to join McClellan. He sent a prompting telegram to that general: “Please do not let him [the enemy] get off without being hurt.” He recognized the battle of Antietam as a substantial, if not a complete victory, and seized the opportunity it afforded him to issue his preliminary proclamation of emancipation on September 22.
For two weeks after the battle of Antietam, General McClellan kept his army camped on various parts of the field, and so far from exhibiting any disposition of advancing against the enemy in the Shenandoah valley, showed constant apprehension lest the enemy might come and attack him. On October 1, the President and several friends made a visit to Antietam, and during the three succeeding days reviewed the troops and went over the various battle-grounds in company with the general. The better insight which the President thus received of the nature and results of the late battle served only to deepen in his mind the conviction he had long entertained—how greatly McClellan’s defects overbalanced his merits as a military leader; and his impatience found vent in a phrase of biting irony. In a morning walk with a friend, waving his arm toward the white tents of the great army, he asked: “Do you know what that is?” The friend, not catching the drift of his thought, said, “It is the Army of the Potomac, I suppose.” “So it is called,” responded the President, in a tone of suppressed indignation, “But that is a mistake. It is only McClellan’s body-guard.”
At that time General McClellan commanded a total force of one hundred thousand men present for duty under his immediate eye, and seventy-three thousand present for duty under General Banks about Washington. It is, therefore, not to be wondered at that on October 6, the second day after Mr. Lincoln’s return to Washington, the following telegram went to the general from Halleck: