12th Corps Headquarters flag
The corps flag is a blue swallowtail with the corps number in red.

 

The monument to the Twelfth Army Corps is south of Gettysburg on Slocum Avenue. A headquarters marker is on Baltimore Pike. (Monument map: South Culp’s Hill)

The Twelfth Corps was commanded at the Battle of Gettysburg by Major General Henry W. Slocum (USMA ’52) a lawyer and state representative from New York whose monument is on Steven’s Knoll.

Monument to the 12th Corps at Gettysburg

From the monument:

Army of the Potomac 
Twelfth Corps
Major General Henry J. Slocum
Brigadier General Alpheus S. Williams

First Division Brigadier General Alpheus S. Williams
Brigadier General Thomas H. Ruger
Second Division Brigadier General John W. Geary
Artillery Brigade Lieutenant Edward D. Muhlenberg
Provost Guard Tenth Maine (Four Companies)

July 1. Marched from near Littlestown to Two Taverns by the afternoon. Hearing the 1st and 11th Corps were engaged at Gettysburg the Corps advanced on the Baltimore Pike. Williams’ Division to a position east of Rock Creek Geary’s Division to the left of Union line extending to the summit of Little Round Top.

July 2. In the morning the Corps took position on the right of 1st Corps on a line extending from the top of Culp’s Hill southeasterly across the low meadow into McAllister’s woods. Later in the day the Corps except Greene’s Brigade was withdrawn to support the left of the Army Johnson’s Confederate Division at night advanced under cover of darkness and took possession of the works on the Corps Line on right of Greene’s Brigade. About midnight the Corps returned and finding Johnson’s Division in possession of the works formed line in front of that Division.

July 3. Before 1 A. M. the artillery of the Corps and Rigby’s Maryland Battery from Reserve Artillery in all 26 guns were so placed as to command the line occupied by Johnson’s Division and at daylight opened fire under cover of which the infantry was advanced and attacked the Confederate position and after a contest lasting seven hours recaptured the works. Many prisoners and 5,000 small arms were captured. In the afternoon the Corps was in readiness to move.

July 4. Gen. Slocum in the morning advanced with a detachment of Ruger’s Division and a battery and found that the Confederates in front had retired.

Casualties Killed 18 Officers 186 Men Wounded 43 Officers 769 Men Captured or Missing 2 Officers 64 Men Total 1082

Monument to the 12th Corps at Gettysburg

From the marker:

Army of the Potomac
12th Corps Headquarters
Major Genegral
Henry W. Slocum
July 1, 2, 3, 4, 1863

were located on Powers Hill
260 yards westerly

Union Major General Henry W. Slocum

Union Major General Henry W. Slocum