English: Allan Pinkerton, President Abraham Lincoln, and Major General John A. McClernand. This photo was taken not long after the Civil War’s first battle on northern soil in Antietam, Maryland on October 3, 1862. Pinkerton was the head of Union Intelligence Services at the time. He also, allegedly, foiled an assassination attempt against Lincoln. His wartime work was critical in Pinkerton’s development, which he later used to pioneer the American private detective industry when he formed the Pinkerton National Detective Agency.