English: Allan Pinkerton, President Abraham Lincoln, and Major General John A. McClernand. This photo and another very similar to it were taken not long after the Civil War’s first battle on northern soil in Antietam, Maryland on October 3, 1862. In his role as head of Union Intelligence Services during the war, Pinkerton foiled an assassination attempt against Lincoln. His wartime work was critical in raising Pinkerton’s profile and helping to bolster the reputation of his Pinkerton National Detective Agency, which pioneered the American private detective industry.