Alexander Gardner / U.S. Library of Congress

Abraham Lincoln at Antietam

Allan Pinkerton, President Abraham Lincoln, and Maj. Gen. John A. McClernand in Antietam, Maryland, on October 3, 1862 -- a few weeks after the Battle of Antietam. Photograph by Alexander Gardner. Pinkerton, founder of his namesake detective agency, was at this time the head of the Union's intelligence services.

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