ANTIETAM, in-tetam, The Battle of, fought on 17 Sept. 1862, in Maryland; sometimes called the battle of Sharpsburg. It was one of the decisive engagements of the Civil War, as it ended the first Confederate attempt at invasion of the North, though tacti cally a drawn battle. Lee's army of about 50,000 crossed the Potomac near Leesburg, some 30 miles above Washington, and concentrated around Frederick, about 40 miles from Wash ington and 20 miles from the Pennsylvania line. When it became known that Lee had crossed into Maryland General McClellan, in command of the Army of the Potomac, pushed forward several corps with the left on the Po tomac.
Meanwhile Lee had ordered a movement on Harper's Ferry (q.v.), in his rear at the junc tion of the Potomac and Shenandoah. It was a valuable defense against invasion through the Valley of Virginia, but once the Confed erates were across it became not only worth less but a trap. Nevertheless General Halleck ordered it held; and Lee grasped the chance of capturing its defenders (with the Martins burg outpost) and opening up his communica tions at once. This involved dividing his army for days, with a much superior force two or three days' march off, but he gauged his foes justly enough to take the risk. The operation was effected with great skill and success and accurate co-ordination; but it took longer than Lee expected, and a mischance befell which should have undone him. A copy of his order fell into McClellan's hands on the 13th, telling him of the dividing of that army not far off, its object, the position of the separate detachments, and the premium on expedition; but the oppor tunity was lost to the Federals through Mc Clellan's unexplained delay in giving orders for an advance.
Lee, advised by the night •of the 13th at Hagerstown that McClellan was advancing on South Mountain, marched back to resist his advance but was defeated and on the night of the 14th fell back to Sharpsburg, a few miles southwest of Boonsboro, as the nearest strong position for his Harper's Ferry detachments to rejoin him, as on the flank and rear of any force menacing Maryland Heights which they occupied, and a very defensible position in it self. Six or seven miles above the Heights the
Potomac receives Antietam Creek, through a ravine, with banks rising on the west to a low ridge having wooded patches, ledges, stone and wooden fences, cornfields, etc., as natural bulwarks, and sloping on the western side to the Potomac_ Two and a half miles above the confluence a• sharp eastern bend of the Potomac brings it within two and one-half miles of the Antietam; and at about the centre of the peninsula thus formed lies Sharpsburg, in a hollow on the western slope. Lee's line, about three miles along the crest, was to rest one flank on an elevation near the Potomac, with the village in the rear centre, and a secure retreat by the Shepherdstown ford of the Potomac in the rear.
The Union troops having forced South Mountain (q.v.) by two sharp battles on the 14th, the main body issued therefrom next morning, marched the eight miles to Antietam Creek and formed in line along the east ridge. By afternoon some 50,000 troops were opposed to about 30,000 under Lee, with Longstreet and D. H. Hill. Late in the afternoon McClellan came up, was received with immense en thusiasm and decided that it was too late to attack that day. On the 16th Jackson and Walker had joined Lee with fully 10,000 more. The chance of splitting the Confederate army being now lost, McClellan waited for his am munition and supply trains to arrive and or dered no attack save of Hooker's corps on the right late in the afternoon. A list of divisions and commanders now becomes requisite for brevity and intelligibility in describing the battle.