President Abraham Lincoln, Gen. J. A. McClernand (on left) and "Major Allen" (Pinkerton) at Antietam, Md., site of the bloodiest single-day battle of the Civil War. The man who called himself Major Allen was in fact Allen Pinkerton, who in 1861 was instrumental in founding the U.S. Secret Service. It was at this time that he assumed the name "E. J. Allen." (September 1862) Mathew Brady. National Archives, Brady Collection