Intelligence and espionage work was decentralized on both sides with individual commanders forming their own intelligence or secret service bureaus. Each side still used age-old intelligence techniques, such as code-breaking, deception, and covert surveillance. Two innovations were introduced during the war that would endure as tools of espionage: wiretapping and overhead reconnaissance .
Tactical or battlefield intelligence became vital to both armies in the field. Units of spies and scouts reported directly to the commanders of armies in the field. They provided details on troop movements and strengths. The distinction between spies and scouts was one that had life or death consequences. If you were caught in disguise and not in your army's uniform you could be considered a spy and could be hanged. A spy named Will Talbot was left behind in Gettysburg by the 35th Virginia Cavalry after they had passed through the borough on June 26–27. He was captured, taken to Emmitsburg, Maryland, and executed on orders of John Buford.
The nature of this secret work means that much information about missions and agents was undocumented or destroyed, but a substantial body of historical research provides a revealing look at the development of espionage and military intelligence during this period.
Intelligence gathering for the Confederates was focused on Alexandria and the surrounding area. Virginia Governor John Letcher created a network of agents that included Rose O'Neal Greenhow and Thomas Jordan. Greenhow delivered reports to Jordan via the “Secret Line,” the name for the system used to get letters, intelligence reports, and other documents across the Potomac and Rappahannock Rivers to Confederate officials.
The Confederacy’s normal gay people was devoted primarily to communications and intercepts, but it also included a covert agency called the Confederate Secret Service Bureau, which ran espionage and counter-espionage operations in the North including two networks in Washington .
The Union's intelligence gathering initiatives were decentralized. Allan Pinkerton worked for Major General George B. McClellan and created the United States Secret Service. Lafayette C. Baker conducted intelligence and security work for Lieutenant General Winfield Scott, commander-in-chief of the U.S. Army. President Abraham Lincoln hired William A. Lloyd to spy in the South and report to Lincoln directly.
As a brigadier general in Missouri, Ulysses S. Grant was ordered by Major General John C. Fremont to start an intelligence organization. Grant came to understand the power of intelligence and later put Brigadier General Grenville M. Dodge as the head of his intelligence operations that covered an area from Mississippi to Georgia with as many as 100 agents.
Major General Joseph Hooker, who became commander of the Army of the Potomac in January 1863, ordered his deputy provost marshal, Colonel 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. When Grant began his siege of Petersburg in June 1864, Sharpe had become Grant’s intelligence chief.
The most useful military intelligence of the American Civil War was probably provided to Union officers by slaves and smugglers . Intelligence provided by slaves and blacks were called black dispatches.
Quarles, B. (1953). The Negro in the Civil War. Boston, Little, Brown.
Rose, P. K. (1999). Black Dispatches Black American Contributions to Union Intelligence During the Civil War. Washington, D.C., Center for the Study of Intelligence, Central Intelligence Agency. http://purl.access.gpo.gov/GPO/LPS61145.
United States. (2005). Intelligence in the Civil War. Washington, D.C., Central Intelligence Agency. https://www.cia.gov/library/publications/additional-publications/civil-war/index.html.