United States Armies in the East

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 general 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.”

Major General Robert Patterson during the Mexican War

A much younger Major General Robert Patterson during the Mexican War

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.

Organization of the Department of Pennsylvania in 1861:

April • MayJune • July

Timeline of the Department of Pennsylvania 1861
April 19 Military District established under Major General Robert Patterson with headquarters in Philadelphia.
April 27 The District of Columbia and Maryland were removed from the Department to become the Department of Washington and the Department of Annapolis by General Order #12. Patterson retained Pennsylvania, Delaware and northern Maryland as the Department of Pennsylvania.
May 1 Patterson raised and equipped 26 regiments; six each at Philadelphia, Harrisburg, Lancaster and York, and two at Chambersburg.
June 12-17 Patterson’s army advanced to occupy Williamsport and Harpers Ferry
July 2 Patterson’s forces at Williamsport, known as the Army of the Upper Potomac, crossed the river at Williamsport and drove back Confederate defenders at Falling Waters.
July 3 Patterson’s army occupied Martinsburg, Virginia, to the cheers of its loyal residents. He found the Baltimore & Ohio Railroad tracks to Harpers Ferry destroyed along with the wreckage of 48 destroyed trains. As the Confederates withdrew up the Shenandoah Valley he settled in and called for supplies and reinforcements.
July 15 Patterson advanced to Bunker Hill. His army had grown to 20,000 men, but most of them were reaching the end of their enlistments within a few days.
July 18 Patterson marched east to Charlestown.
July 19 General Order #46 honorably discharged General Patterson at the end of his term of service on the 27th, along with General Cadwallader. Most remaining forces in the department were transferred to the Department of the Shenandoah, which was 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 moved 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 Nathaniel P. Banks replaced General Patterson and took command of the Department of the Shenandoah.