When the Civil War broke out, Alan Pinkerton (1819 - 1884) was given charge of the Union Intelligence Service, having previously gained tremendous credibility as a detective in Chicago. It was at this post, early in the war, that he was assigned a task by General George McClellan (1826 - 1885) to proceed south of the Ohio River in order to gain a more thorough understand as to the loyalties of those people. Pinkerton first wrote about this mission in his Civil War memoir, A Spy of the Rebellion.

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