Department of Pennsylvania (April - July 1861)

The department was created on April 19, 1861 to consist of the states of Pennsylvania, Delaware, and Maryland and the District of Columbia. It was placed placed under the command of Pennsylvania Militia Major General Robert Patterson, a Mexican War colleage and close friend of General Winfield Scott. Patterson's militia commission was transferred from Pennsylvania to the United States Army for three months. Patterson's force is occasionally referred to as the "Army of the Shenandoah" or the "Army of the Upper Potomac."

Patterson's original mission was to open and hold the communications routes between the northern states and Washington. When that was largely acomplished by Union Brigadier General Benjamin Butler's occupation of Baltimore, Patterson was then tasked with holding Confederate General Joseph Johnston's forces in the Shenandoah Valley and preventing him from sending reinforcements to Beauregard at Manasas. He failed, and Johnston's reinforcements were decisive in the Confederate victory at Bull Run.

Although Patterson was never censured, his commission was allowed to expire at the end of its term. At the same time most of his army was mustering out at the end of their ninety day enlistments. A new army of long-service regiments would be built up almost from scratch in the newly-designated Department of the Shenandoah under Major General Nathaniel Banks.

Major General
Robert Patterson
during the Mexican War

Order of Battle for the Department of Pennylvania

Department of Pennsylvania Order of Battle, April 1861

Department of Pennsylvania Order of Battle, May 1861

Department of Pennsylvania Order of Battle, June 1861

Department of Pennsylvania Order of Battle, July 1861

Timeline of the Department of Pennsylvania
April 19, 1861	Military District established under Major General Robert Patterson with headquarters in Philadelphia.
April 27	The District of Columbia and Maryland are removed from the Department to become the Military District of Washington and the Military District of Annapolis by General Order #12. Patterson retains Pennsylvania, Delaware and northern Maryland as the Department of Pennsylvania.
May 1	Patterson has raised and equipped 26 regiments; six each at Philadelphia, Harrisburg, Lancaster and York, and two at Chambersbug.
June 12-17	Patterson's army advances to occupy Williamsport and Harpers Ferry
July 2	Patterson's forces at Williamsport, known as the Army of the Upper Potomac, crosses the river at Williamsport and drives back Confederate defenders at Falling Waters.
July 3	Patterson's army ocupies Martinsburg, Virginia, to the cheers of its loyal residents. He finds the Baltimore & Ohio Railroad tracks to Harpers Ferry destroyed along with the wreckage of 48 destroyed trains. As the Confederates withdraw up the Shenandoah Valley he settles in and calls for supplies and reinforcements.
July 15	Patterson advances to Bunker Hill. His army has grown to 20,000 men, but most of them will be reaching the end of their enlistments within a few days.
July 18	Patterson marches east to Charlestown.
July 19	General Order #46 honorably discharges General Patterson at the end of his term of service on the 27th, along with General Cadwallader. Most remaining forces in the department will be transferred to the Department of the Shenandoah, which is created to consist of the Valley of Virginia, the Counties of Washington and Allegheny in Maryland and such parts of Virginia as may be covered by the army in its operations.
July 21	Patterson's army moves to Harpers Ferry
July 25	Department consolidated with the Department of Northeastern Virginia and the Department of Maryland to form the Military District of the Potomac.
July 27, 1861	Nathaniel P. Banks replaces General Patterson and takes command of the Department of the Shenandoah,

Department of Pennsylvania (April - July 1861)

Order of Battle for the Department of Pennylvania

Timeline of the Department of Pennsylvania