Why was a Pennsylvania brigade held in Washington during Gettysburg?

During the American Civil War one brigade of the Pennsylvania Reserves stayed in the Washington defenses during the Gettysburg campaign. Why did this happen?

After Gettysburg this remaining brigade was split, with two of the regiments being sent to West Virginia (the 3rd and 4th Pennsylvania Reserves.) Why did this happen to these two regiments? I have searched for this information in the regimental histories but cannot find the answer.

Thank you,
Bill Schoenfeld

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Dear Mr. Schoenfeld,

Sometimes a little research on a unit, combined with a knowledge of the “big picture,” can yield an answer even if the unit history doesn’t declare it outright. In the case of the 3rd Brigade, Pennsylvania Reserves, its four regiments were withdrawn from the Army of the Potomac and relegated to the defense of Washington, D.C., for two reasons. First, some of them had suffered heavy losses at Fredericksburg—128 casualties in the case of the 3rd Pennsylvania Reserves and 28 dead, 86 wounded and 22 taken prisoner from the 8th Pennsylvania Reserves. In the wake of that, they were sent to Washington to rest and recruit replacements for their losses. At the same time, on February 2, 1863, the brigade became part of the XXII Corps, under Maj. Gen. Samuel R. Heinzelman, defending the capital. This became more than just a cushy behind-the-lines job in June 1863, when General Robert E. Lee led his Army of Northern Virginia on its second invasion of the north. Concurrent with the main body’s advance were numerous Rebel thrusts in various directions, such as Maj. Gen. John B. Gordon’s advance as far east as Wrightstown, Pa., halting short of Columbia only because the retreating local militia burned the bridge across the Susquehanna River. Raiding throughout Pennsylvania and Maryland, Maj. Gen. J.E.B. Stuart claimed after one successful skirmish in Westminster, Md., that only fatigued horses prevented his cavalry from following their fleeing Yankee counterparts right into Washington and taking President Abraham Lincoln prisoner. However valid or not Stuart’s boast, both Washington and Pennsylvania perceived a very real threat and the 3rd Brigade, Pennsylvania Reserves was playing a very necessary role, even if it didn’t make Gettysburg when Lee was finally brought to battle.

Afterward, in May 1864 the 8th and 9th Pennsylvania Reserves were attached to the 3rd Division of the V Corps, Army of the Potomac for the Overland Campaign, but the 3rd and 4th regiments were given another assignment as part of Brig. Gen. George Crook’s Army of West Virginia: a move to cut off the last major junction of the Tennessee-Virginia Railroad in Pulaski County in western Virginia. This was all part of a comprehensive offensive that Lt. Gen. Ulysses S. Grant was pursuing not merely against Lee, but on several vital fronts from the James to Georgia. Again, this was no rest tour for the 3rd and 4th Pa. Reserves, which had a major fight on their hands against Brig. Gen. Albert G. Jenkins’ force at Cloyd’s Mountain on May 9.

Hope this information helps sort out the reasons for you.

Sincerely,