701262 Scouts and Guides to the Army of the Potomac, 1862 (b/w photo) by Gardner, Alexander (1821-82); Private Collection; (add.info.: Photograph from the book "Original Photographs Taken on the Battlefields During the Civil War," by Mathew Brady and Alexander Gardner. The Army of the Potomac was the major Union Army in the Eastern Theater of the American Civil War. The individuals in this group were attached to the Secret Service Department of the Army of the Potomac when conducted by Major Allen Pinkerton. ); The Stapleton Collection; American, out of copyright.