Maj. Jed. Hotchkiss, Confederate Military History, a library of Confederate States Military History: Volume 3, Virginia
Clement Anselm Evans, Ed.

[341] of his army, and as he wrote to the President, he said: ‘Our ranks are very much diminished—I fear from a third to one-half of the original numbers.’ Still more disturbing was the news that reached him on the evening of that day. This was of the rapid approach of McClellan in force on the National road toward Hill's position on the South mountain, and toward that of Mc-Laws on the Potomac north of Harper's Ferry and Maryland heights. He knew McClellan's military characteristics, not only from his personal knowledge of him before the existing war, but especially from his doings in leading the great army of the Potomac, in the ‘on to Richmond’ and in the ‘back to Washington,’ and therefore could not account for his unusual diligence in pursuing him westward from the Monocacy, to reach which from Washington he had been marching with great caution. McClellan's report, published by the Federal government the following winter, furnished the explanation. On the morning of this same Saturday, the 13th of September, after McClellan had occupied Frederick on the 12th, there was handed him an official copy of Lee's order No. 191, which revealed, in detail, the entire plan of the pending campaign, and showed him, at a glance, how Lee's knights and castles on the military chess-board were disposed, and that a rare opportunity was offered for falling upon his greatly weakened left rear and crushing that before he could gather his scattered forces to his aid, as McClellan had the great advantage of the far shorter line of approach over one of the best roads. More than this; it showed that the dreaded Jackson was too far away to participate in an early combat. Ardent to retrieve his military reputation, and, above all things, anxious to do something that would warrant his unauthorized assumption of command, McClellan at once hastened his main body in pursuit of Lee, and urged Franklin forward with his corps, to harass the rear of McLaws and hold him away from the battle he proposed to make with Lee.

It is now known that two copies of Lee's order were sent to D. H. Hill, who had been made subject to Jackson's command previous to the encampment at Frederick. Jackson, always cautious and himself never giving written orders that would furnish information as to his movements, had, on the receipt of Lee's order, made, with his