The monument to the Third Delaware Volunteers at Antietam is the West Woods in Philadelphia Brigade Park. (39.479068° N, 77.749527° W; map) It was dedicated on May 30, 1964.
The 3d Delaware was commanded at the Battle of Antietam by Colonel Arthur Maginnis. He was wounded, and Captain William J. McKaig took over. The regiment had suffered heavily at the Second Battle of Bull Run two weeks prior to Antietam, and brought only 125 men to the field.
From the monument:
3rd Delaware Volunteers
Major Arthur Maginnis
3rd Brigade, 2nd Division, XII Corps
This regiment, worn down from active service in the second Manassas Campaign, went into action Sept. 17, 1862 with only five officers and 120 men. It started out on the right of the Union line in front of the West Woods. After heavy action along the Hagerstown Pike, it helped repulse Confederate counter attack following rout of Sedgewick's Division. Final position of the regiment, 65 yards North of this point.
Killed 1 5
Wounded 2 9
Total 17 of 125 engaged
Erected by the Delaware Civil War Centennial Commission May 30, 1964