American Civil War spies < nowiki > < / nowiki > Tactical or battlefield intelligence became very vital to both armies in the field during the American Civil War . 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 a suspect was seized while in disguise and not in his army ' s uniform , the sentence was often to be hanged . A spy named Will Talbot , a member of the 35th Battalion , Virginia Cavalry , was left behind in Gettysburg after his battalion had passed through the borough on June 26 – 27 , 1863 . He was captured , taken to Emmitsburg , Maryland , and executed on orders of Brig . Gen . John Buford . Confederate . Intelligence gathering for the Confederates was focused on Alexandria , Virginia , 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 Signal Corps 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 . Union . The Union ' s intelligence gathering initiatives were decentralized . Allan Pinkerton worked for Maj . Gen . 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 Alvin Lloyd to spy in the South and report to Lincoln directly . As a brigadier general in Missouri , Ulysses S . Grant was ordered by Maj . Gen . John C . Frémont to start an intelligence organization . Grant came to understand the power of intelligence and later put Brig . Gen . Grenville M . Dodge as the head of his intelligence operations that covered an area from Mississippi to Georgia with as many as one hundred secret agents . 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 . 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 smuggler s . Intelligence provided by slaves and blacks were called black dispatches .