Jackson's corps had been sent on one of his famous "foot cavalry" expeditions to sweep the Federal garrison from Martinsburg, surround and capture Harper's Ferry. McClellan at once moved a division of his army to crush the small command Lee had stationed at South Mountain to guard Jackson's movement.
McClellan threw his men against this little division of the Confederates and attempted to force his way to the relief of Harper's Ferry. The battle raged with fury until nine o'clock at night. Their purpose accomplished Lee withdrew them to his new position at Sharpsburg to await the advent of Jackson.
The "foot cavalry" had surrounded Harper's Ferry, assaulted it at dawn and in two hours the garrison surrendered. Thirteen thousand prisoners with their rifles and seventy-three pieces of artillery fell into Jackson's hands. Leaving General A. P. Hill to receive the final surrender of the troops Jackson set out at once for Sharpsburg to join his army with Lee's.
The Southern Commander had but forty thousand men with which to meet McClellan's ninety thousand, but at sunrise on September seventeenth, his batteries opened fire and the bloodiest struggle of the Civil War began. Through the long hours of this eventful day the lines of blue and gray charged and counter-charged across the scarlet field. When darkness fell neither side had yielded. The dead lay in ghastly heaps and the long pitiful wail of the wounded rose to Heaven.
Lee had lost two thousand killed and six thousand wounded. McClellan had lost more than twelve thousand. His army was so terribly shattered by the bloody work, he did not renew the struggle on the following day. Lee waited until night for his assault and learning that reënforcements were on the way to join McClellan's command withdrew across the Potomac.
It was a day later before Lee's movements were sufficiently clear for McClellan to claim a victory.
On September nineteenth, he telegraphed Washington:
"I do not know if the enemy is falling back or recrossing the river. We may safely claim the victory as ours."
Abraham Lincoln hastened to take advantage of McClellan's claim to issue his Emancipation Proclamation. And yet so utter had been the failure of his general to cope with Lee and Jackson, the President of the United States relieved McClellan of his command.