William Swinton, Campaigns of the Army of the Potomac

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Upon the retirement of the Confederates on the morning of the 15th, McClellan pushed forward his whole army in pursuit; but after a few miles' march, the heads of the columns were brought to a sudden halt at Antietam Creek, a rivulet that, running obliquely to the course of the Potomac, empties into it six miles above Harper's Ferry. On the heights crowning the west bank of this stream, Lee, with what force he had in hand, took his stand to oppose McClellan's pursuit, and form a point of concentration for his scattered columns.

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II. the battle of Antietam.

Whatever ulterior purposes Lee may have had touching the prosecution of the Maryland invasion, affairs had so worked together that it had become now absolutely necessary for him to stand and give battle. Whether he designed abandoning the aggressive and repassing the Potomac, or purposed manoeuvring by the line of Western Maryland towards Pennsylvania, he was obliged first of all to take up a position on which he might unite his divided forces, closely pressed by the advancing Union columns, and receive the attack of his antagonist. The circumstances were such that a battle would necessarily decide the issue of the invasion.

It was late in the afternoon of the 15th when the Army of the Potomac drew up on the left bank of Antietam Creek, on the opposite side of which the Confederate infantry was seen ostentatiously displayed. The day passed in observation of the position, and next morning that moiety of the Confederate force that had been engaged in the investment of Harper's Ferry rejoined Lee. The Confederate commander formed his troops on a line stretched across the angle formed by the Potomac and Antietam; and as the Potomac here makes a sharp curve, Lee was able to rest both