nine o'clock at night
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 .