English: Abraham Lincoln (centre) in front of tent flanked by two other men Allan Pinkerton, President Abraham Lincoln, and Major General John Alexander McClernand. 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. Photograph by Alexander Gardner, 3 October 1863.