American Civil War spies

Units of spies and scouts reported directly to the commanders of armies in the field, providing details on troop movements and strengths.

[4] Maj. Gen. Joseph Hooker, who became commander of the Army of the Potomac in January 1863, ordered his deputy provost marshal, Col. George H. Sharpe, to create a unit to gather intelligence.

Sharpe set up what he called the Bureau of Military Information and was aided by John C. Babcock, who had worked for Allan Pinkerton and had made maps for George B. McClellan.

Sharpe's bureau produced reports based on information collected from agents, prisoners of war, refugees, Southern newspapers, documents retrieved from battlefield corpses, and other sources.

[4] African American slaves and free persons provided valuable intelligence supporting Union military operations, often exploiting their ability to move across lines without attracting attention.