The defeat and dispersion of the army of General Pope on the last day of August seemed to have opened Pennsylvania to the Confederates.  On the 15th of September, a fortnight afterward, General McClellan, at the head of a new army, raised in large measure by the magic of his name, had pursued the victorious Confederate, checked his further advance, and, forcing him to abandon his designs of invasion, brought him to bay a hundred miles from the capital.  This was generalship, it would seem, in the true acceptation of the term, and McClellan, harassed and hampered by the authorities, who looked but coldly upon him, could say, with Coriolanus, “Alone I did it.”

Lee was thus compelled to give up his movement in the direction of Pennsylvania, and concentrate his army to receive the assault of General McClellan.  Jackson, marching with his customary promptness, joined him with a portion of the detached force on the next day (September 16th), and almost immediately those thunders which prelude the great struggles of history began.

General Lee had drawn up his army on the high ground west of the Antietam, a narrow and winding stream which flows, through fields dotted with homesteads and clumps of fruit and forest trees, to the Potomac.  Longstreet’s corps was posted on the right of the road from Sharpsburg to Boonsboro, his right flank guarded by the waters of the stream, which here bends westward; on the left of the Boonsboro road D.H.  Hill’s command was stationed; two brigades under General Hood were drawn up on Hill’s left; and when Jackson arrived Lee directed him to post his command on the left of Hood, his right resting on the Hagerstown road, and his left extending backward obliquely toward the Potomac, here making a large bend, where Stuart with his cavalry and horse-artillery occupied the ground to the river’s bank.

This arrangement of his troops was extremely judicious, as the sequel proved.  It was probable that General McClellan would direct his main attack against the Confederate left, with the view of turning that flank and hemming in the Southern army, or driving it into the river.  By retiring Jackson’s left, Lee provided for this contingency, and it will be seen that the design attributed by him to his adversary was that determined upon.

General McClellan occupied the ground on the eastern bank of the Antietam.  He had evidently massed his forces opposite the Confederate left, but a heavy order of battle stood opposite the centre and right of Lee, where bridges crossed the stream.

The respective numbers of the adversaries can be stated with accuracy.  “Our forces at the battle of Antietam,” said General McClellan, when before the committee of investigation afterward, “were, total in action, eighty-seven thousand one hundred and sixty-four.”

General Lee says in his report:  “This great battle was fought by less than forty thousand men on our side.”

Colonel Walter H. Taylor, a gentleman of the highest character, and formerly adjutant-general of the army, makes the Confederate numbers somewhat less.  In a memorandum before the writer, he says: