On August 17, 1861, Lieutenant General Winfield Scott issued General Orders, No. 15 (Headquarters of the Army) announcing the creation of the Department of the Potomac commanded by Major General George B. McClellan.
General Orders, No. 15
Headquarters of the Army
Washington,August 17, 1861
The Departments of Washington and Northeastern Virginia will be united into one, to which will be annexed the Valley of the Shenandoah, the whole of Maryland and of Delaware, to be denominated the Department of the Potomac, under Major-General McClellan – headquarters Washington – who will proceed to organize the troops under him into divisions and independent brigades.
By command of Lieutenant-General Scott
E. D. TOWNSEND
Assistant Adjutant General.