I immediately took steps to carry out these orders, and sent an aide to General Pope with the following letter:

HEADQUARTERS,
Washington, September 2, 1862

Major General John Pope,

Commanding Army of Virginia:

GENERAL: General Halleck instructed me to repeat to you the order he sent this morning to withdraw your army to Washington without unnecessary delay. He feared that his messenger might miss you, and desired to take this double precaution. In order to bring troops upon ground with which they are already familiar, it would be best to move Porter's corps upon Upton's Hill, that it my occupy Hall's Hill, &c.; McDowell's to Upton's Hill; Franklin's to the works in front of Alexandria; Heintzelman's to the same vicinity; Couch to Fort Corcoran, or, if practicable, to the Chain Bridge; Sumner either to Fort Albany or to Alexandria, as may be most convenient.

In haste, general, very truly, yours,

GEO. B. McCLELLAN,

Major-General, U. S. Army.

In the afternoon I crossed the Potomac and rode to the front, and at Upton's Hill met the advance of McDowell's corps, and with it Generals Pope and McDowell. After getting what information I could from them, I sent the few aides at my disposal to the left, to give instructions to the troops approaching in the direction of Alexandria, and, hearing artillery firing in the direction of the Vienna and Langley road, by which the corps of Sumner, Porter, and Sigel were returning, and learning from General Pope that Sumner was probably engaged, I went with a single aide and three orderlies by the shortest line to meet that column. I reached the column after dark, and proceeded as far as Lewinsville, where I became satisfied that the rear corps (Sumner's) would be able to reach its intended position without any serious molestation. I therefore indicated to Generals Porter and Sigel the positions they were to occupy, sent instructions to General Sumner, and at a late hour of the night returned to Washington.

Next day I rode to the front of Alexandria, and was engaged in rectifying the positions of the troops and giving orders necessary to secure the issuing of the necessary supplies, &c. I felt sure on this day that we could repulse any attack made by the enemy on the south side of the Potomac.

On the 3rd the enemy had disappeared from the front of Washington, and the information which I received induced me to believe that he intended to cross the Upper Potomac into Maryland. This materially changed the aspect of affairs and enlarged the sphere of operations; for, in case of a crossing in force, an active campaign would be necessary to cover Baltimore, prevent the invasion of Pennsylvania, and clear Maryland. I therefore on the 3rd ordered the Second and Twelfth Corps to Tennallytown, and the Ninth Corps to a point on the Seventh street road near Washington, and sent such cavalry as was available to the fords near Poolesville, to watch and impede the enemy in any attempt to cross in that vicinity.

On September 5 the Second and Twelfth Corps were moved to Rockville, and Couch's division (the only one of the Fourth Corps that had been brought from the Peninsula) to Offutt's Cross-Roads.

On the 6th the First and Ninth Corps were ordered to Leesborough; the Sixth Corps and Sykes' division of the Fifth Corps to Tennallytown.

On the 7th the Sixth Corps was advanced to Rockville, to which place my headquarters were moved on the same day.

All the necessary arrangements for the defense of the city under the new condition of things had been made, and General Banks was left in command, having received his instructions from me.