On April 4, 1862, President Lincoln created the Department of the Rappahannock from the larger Department of the Potomac.,[1] The I Corps from the Army of the Potomac was detached to form the basis of the new department.[1] Its territory consisted of Virginia “east of the Blue Ridge and west of the Potomac River, the Fredericksburg and Richmond Railroad, including the District of Columbia and the country between the Potomac and the Patuxent Rivers.” The commander of the I Corps, Irvin McDowell, was promoted to Major General and authorized to command the department.[2] Northeastern Virginia was added to the department on June 1, 1862. The Department of the Shenandoah absorbed the Piedmont District and Bull Mountain Range on June 1, 1862.[1] The District of Columbia became the Department of Washington on June 26, 1862.[1]
From June 26, 1862 to September 12, 1862, the troops of the Department of the Rappahannock were designated the III Corps of the Army of Virginia.[1] On September 12, 1862, the III Corps became the I Corps of the Army of the Potomac.[5] Brigadier General James B. Ricketts commanded III Corps on September 5–6, 1862.[6] III Corps, then I Corps, was commanded by Major General Joseph Hooker from September 6, 1862 to September 17, 1862.[7]