It turned out that the second battle of Bull Run had by no means so seriously disorganized the Union army as was reported, and that Washington had been exposed to no real danger. The Confederate army hovered on its front for a day or two, but made neither attack nor demonstration. Instead of this, Lee entered upon a campaign into Maryland, hoping that his presence might stimulate a secession revolt in that State, and possibly create the opportunity successfully to attack Baltimore or Philadelphia.
Pope having been relieved and sent to another department, McClellan soon restored order among the troops, and displayed unwonted energy and vigilance in watching the movements of the enemy, as Lee gradually moved his forces northwestward toward Leesburg, thirty miles from Washington, where he crossed the Potomac and took position at Frederick, ten miles farther away. McClellan gradually followed the movement of the enemy, keeping the Army of the Potomac constantly in a position to protect both Washington and Baltimore against an attack. In this way it happened that without any order or express intention on the part of either the general or the President, McClellan's duty became imperceptibly changed from that of merely defending Washington city to that of an active campaign into Maryland to follow the Confederate army.
This movement into Maryland was begun by both armies about September 4. On the thirteenth of that month McClellan had reached Frederick, while Lee was by that time across the Catoctin range at Boonsboro', but his army was divided. He had sent a large part of it back across the Potomac to capture Harper's Ferry and Martinsburg. On that day there fell into McClellan's hands the copy of an order issued by General Lee three days before, which, as McClellan himself states in his report, fully disclosed Lee's plans. The situation was therefore, as follows: It was splendid September weather, with the roads in fine condition. McClellan commanded a total moving force of more than eighty thousand; Lee, a total moving force of forty thousand. The Confederate army was divided. Each of the separate portions was within twenty miles of the Union columns; and before half-past six on the evening of September 13, McClellan had full knowledge of the enemy's plans.
General Palfrey, an intelligent critic friendly to McClellan, distinctly admits that the Union army, properly commanded, could have absolutely annihilated the Confederate forces. But the result proved quite different. Even such advantages in McClellan's hands failed to rouse him to vigorous and decisive action. As usual, hesitation and tardiness characterized the orders and movements of the Union forces, and during the four days succeeding, Lee had captured Harper's Ferry with eleven thousand prisoners and seventy-three pieces of artillery, reunited his army, and fought the defensive battle of Antietam on September 17, with almost every Confederate soldier engaged, while one third of McClellan's army was not engaged at all and the remainder went into action piecemeal and successively, under such orders that coöperative movement and mutual support were practically impossible. Substantially, it was a drawn battle, with appalling slaughter on both sides.
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:
"I am instructed to telegraph you as follows: The President directs that you cross the Potomac and give battle to the enemy, or drive him south. Your army must move now while the roads are good. If you cross the river between the enemy and Washington, and cover the latter by your operation, you can be reinforced with thirty thousand men. If you move up the valley of the Shenandoah, not more than twelve thousand or fifteen thousand can be sent to you. The President advises the interior line, between Washington and the enemy, but does not order it. He is very desirous that your army move as soon as possible. You will immediately report what line you adopt, and when you intend to cross the river; also to what point the reinforcements are to be sent. It is necessary that the plan of your operations be positively determined on before orders are given for building bridges and repairing railroads. I am directed to add that the Secretary of War and the general-in-chief fully concur with the President in these instructions."
This express order was reinforced by a long letter from the President, dated October 13, specifically pointing out the decided advantages McClellan possessed over the enemy, and suggesting a plan of campaign even to details, the importance and value of which was self-evident.