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 cooeperative movement and mutual support were practically impossible.  Substantially, it was a drawn battle, with appalling slaughter on both sides.