A pretty widely reproduced picture of Lincoln with Allan Pinkerton. Pinkerton is well known for his founding of the Pinkerton Detectives, but he was also a crucial part of the Union military as head of the Union Intelligence Division. He’s also one of the few civilian members of the Military Intelligence Hall of Fame. More at Wikipedia.
It would be interesting to see some followup on spying done during the civil war. Was it a crucial part of the war strategy? Also, was it hard to spy on fellow citizens vs. enemy countries.