Department of Pennsylvania ( American Civil War ) The Department of Pennsylvania ( or Army of the Shenandoah or General Patterson ' s Army ) was a large military unit in the Union Army at the outset of the American Civil War . Established on April 27 , 1861 , its territory consisted of Pennsylvania , Delaware , and all of Maryland not embraced in the Department of Annapolis ( later renamed Department of Maryland ) and the Department of Washington . It later absorbed the Department of Maryland , on July 25 , 1861 . On August 24 , 1861 it was merged into the Department of the Potomac . On December 1 , 1864 , the Department of Pennsylvania was recreated , with the merging of the Department of the Susquehanna with the Department of the Monongahela . It operated until June 27 , 1865 under the Middle Military Division . First Department of Pennsylvania . It was under the command of Major General Robert Patterson , its manpower mainly consisted of three - month troops from the states of Pennsylvania and New York . After achieving a tactical victory at the Battle of Hoke ' s Run on July 2 and contributing indirectly to the Union disaster at the First Battle of Bull Run on July 21 , its unexpired regiments and commanders were absorbed into the " Department of the Shenandoah " under the command of Major General Nathaniel P . Banks on July 25 , 1861 . Pennsylvania Reserves . When President Abraham Lincoln called for 75 , 000 volunteers in the spring of 1861 , the Commonwealth of Pennsylvania produced more than twice its quota . For political reasons the U . S . Secretary of War , Simon Cameron , rejected fifteen regiments for immediate service , and they became known as the 1st through 15th Pennsylvania Reserves . Pennsylvania elected to retain , organize , train and equip them at its own expense . < br > These fifteen regiments were unavailable either to Patterson in the Shenandoah Valley or to McDowell at First Bull Run . " Within four days after the disaster at Bull ' s Run , eleven regiments of this fine body of men ( armed , drilled , clothed , equipped , and in all respects ready for active service ) were in Washington , " Governor Andrew G . Curtin reminded the State Legislature . Battle of Hoke ' s Run . On July 2 , 1861 , Major General Robert Patterson ' s army , called the " Department of Pennsylvania " , crossed the Potomac River near Williamsport , Maryland , and marched on the main road toward Martinsburg , Virginia . Near Hoke ' s Run , Union forces clashed with Confederate forces and slowly drove them back toward Winchester . Withdrawing before Patterson ' s larger force , Colonel Thomas J . Jackson accomplished his orders to delay the Federal advance . The encounter was very brief , and three regiments reported casualties : First Wisconsin Infantry , Eleventh Pennsylvania Infantry , and Fifteenth Pennsylvania Infantry . The Order of Battle shows all units in the organization at its peak size and structure . First Battle of Bull Run . During the afternoon of July 21 , 1861 , the arrival of BG General Joseph E . Johnston reversed the tide of Battle , the ensuing Confederate counterattack turned victory into a rout , and euphoric Northern hopes for a short war were quickly dashed . Lieutenant General Winfield Scott had hoped that Patterson and his Department of Pennsylvania would prevent Johnston from reinforcing Beauregard at Manassas Junction , and when Johnston ' s troops became a major deciding factor , Patterson ' s actions became a subject of controversy . After the fact , Patterson believed : Patterson was honorably discharged on July 27 , 1861 , and never received another commission . He sought to redress the record through every possible avenue , including a personal audience with President Abraham Lincoln , and later published his own version of events .