Harper's Ferry must be captured, and, having effected that result, the whole Confederate force, detached for the purpose, was to follow the main body of this army in the direction of Hagerstown, to take part in the proposed invasion of Pennsylvania.
This excellent plan failed, as will be seen, from no fault of the great soldier who devised it, but in consequence of unforeseen obstacles, and especially of one of those singular incidents which occasionally reverse the best-laid schemes and abruptly turn aside the currents of history.
Jackson and the commanders cooeperating with him moved on September 10th.

General Lee then with his main body crossed the South Mountain, taking the direction of Hagerstown.

Meanwhile, General McClellan had advanced cautiously and slowly, withheld by incessant dispatches from Washington, warning him not to move in such a manner as to expose that city to danger.

Such danger existed only in the imaginations of the authorities, as the army in advancing extended its front from the Potomac to the Baltimore and Ohio Railroad.

General McClellan, nevertheless, moved with very great precaution, feeling his way, step by step, like a man in the dark, when on reaching Frederick City, which the Confederates had just evacuated, good fortune suddenly came to his assistance.