Allan Pinkerton, The spy in the rebellion; being a true history of the spy system of the United States Army during the late rebellion, revealing many secrets of the war hitherto not made public, compiled from official reports prepared for President Lincoln , General McClellan and the Provost-Marshal-General .

General McClellan left Washington on the 7th day of September, and established his headquarters at Rockville, having first made all arrangements for the defense of Washington, and placing General Banks in command of the troops at that place. By this time it was known that the mass of the rebel army had passed up the south side of the Potomac river, in the direction of Leesburg, and that a part of the army had crossed the river into Maryland.

The uncertainty of Lee's intentions greatly distracted the authorities at Washington for the safety of that city, and they were fearful that he would make a feint towards Pennsylvania, and then suddenly seize the opportunity to attack the capital.

Some writers have animadverted freely upon the alleged “slowness” of McClellan's movements up the Potomac, and his “delay” in offering battle to Lee before the latter had time to unite his army and occupy the strong position he held at Antietam; but they persistently ignore the fact that the dispatches from the commander-in-chief at Washington, to McClellan in the field, from the 7th to the 16th of September, were filled with cautions against a too hasty advance, and the consequent impropriety of exposing Washington to an attack. Indeed, it seems evident to me, when I regard the career of the Army