Painting of Gen. Robert Lee and troops during Battle of Fredericksburg
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Though neither the Union nor the Confederacy had a formal military intelligence network during the Civil War, each side obtained crucial information from spying or espionage operations. From early in the war, the Confederacy set up a spy network in the federal capital of Washington, D.C., home to many southern sympathizers. The Confederate Signal Corps also included a covert intelligence agency known as the Secret Service Bureau, which managed spying operations along the so-called “Secret Line” from Washington to Richmond. As the Union had no centralized military intelligence agency, individual generals took charge of intelligence gathering for their own operations. General George B. McClellan hired the prominent Chicago detective Allan Pinkerton to set up the first Union espionage organization in mid-1861.

Located 60 miles south of the Mason-Dixon Line, Washington, D.C. was full of southern sympathizers when the Civil War broke out in 1861. Virginia’s Governor John Letcher, a former congressman, used his knowledge of the city to set up a nascent spy network in the capital in late April 1861, after his state seceded but before it officially joined the Confederacy. Two of the most prominent early recruits were Thomas Jordan, a West Point graduate stationed in Washington before the war, and Rose O’Neal Greenhow, an openly pro-South widow and socialite who was friendly with a number of northern politicians, including Secretary of State William Seward and Massachusetts Senator Henry Wilson.

In July 1861, Greenhow sent coded reports across the Potomac to Jordan (now a volunteer in the Virginia militia) concerning the planned Federal invasion. One of her couriers, a young woman named Bettie Duvall, dressed as a farm girl in order to pass Union sentinels on the Chain Bridge leaving Washington, then rode at high speed to Fairfax Courthouse in Virginia to deliver her message to Confederate officers stationed there. Confederate General P.G.T. Beauregard later credited the information received from Greenhow with helping his rebel army win a surprise victory in the First Battle of Bull Run (Manassas) on July 21.

The Confederate Signal Corps, which operated the semaphore system used for communicating vital information between armies on the field, also set up a covert intelligence operation known as the Secret Service Bureau. Headed by William Norris, the former Baltimore lawyer who also served as chief signal officer for the Confederacy, the bureau managed the so-called “Secret Line,” an ever-changing system of couriers used to get information from Washington across the Potomac and Rappahannock Rivers to Confederate officials in Richmond. The Secret Service Bureau also handled the passing of coded messages from Richmond to Confederate agents in the North, Canada and Europe.

A number of Confederate soldiers, especially cavalrymen, also acted as spies or “scouts” for the rebel cause. Among the most famous were John Singleton Mosby, known as the “Gray Ghost,” who led guerrilla warfare in western Virginia through the latter years of the war, and especially J.E.B. Stuart, the celebrated cavalry officer whom General Robert E. Lee called “the eyes of the army.”

Allan Pinkerton, the founder of his own detective agency in Chicago, had collected intelligence for Union General George B. McClellan during the first months of the Civil War, while McClellan led the Department of Ohio. When President Abraham Lincoln summoned McClellan to Washington late that summer, the general put the detective in charge for intelligence for his Army of the Potomac, and Pinkerton set up the first Union espionage operation in mid-1861. Calling himself E.J. Allen, Pinkerton built a counterintelligence network in Washington and sent undercover agents to ingratiate themselves in the Confederate capital of Richmond. Unfortunately, Pinkerton’s intelligence reports from the field during 1862′s Peninsula Campaign consistently miscalculated Confederate numbers at twice or three times their actual strength, fueling McClellan’s repeated calls for reinforcements and reluctance to act.

Though he called his operation the U.S. Secret Service, Pinkerton actually worked only for McClellan. Union military intelligence was still decentralized at the time, as generals (and even President Lincoln) employed their own agents to seek out information and report back to them. Another prominent Union intelligence officer was Lafayette C. Baker, who worked for the former Union General in Chief Winfield Scott and later for Secretary of War Edwin Stanton. The brave but ruthless Baker was notorious for rounding up Washingtonians suspected of having southern sympathies; he later directed the manhunt for John Wilkes Booth, the actor and Confederate sympathizer who shot and killed Lincoln at Ford’s Theatre in April 1865.

Thanks to her success, Rose O’Neal Greenhow was one of the first Confederate spies targeted by Allan Pinkerton. Shortly after the southern victory in the First Battle of Bull Run, Pinkerton put Greenhow under surveillance and subsequently arrested her. Imprisoned in the Old Capitol Prison, she was released in June 1862 and sent to Richmond. Belle Boyd, another famous southern belle-turned-Confederate spy, helped smuggle intelligence to General Stonewall Jackson during his Shenandoah Valley campaign in 1862. Like the Confederacy, the Union also made use of female spies: Richmond’s Elizabeth Van Lew, known as “Crazy Bett,” risked her life running an espionage operation out of her family’s farm, while Sarah Emma Edmonds disguised herself as a black slave to enter Confederate camps in Virginia.

The British-born Timothy Webster, a former police officer in New York City, became the Civil War’s first double agent. Sent by Pinkerton to Richmond, Webster pretended to be a courier on the Secret Line, and managed to gain the trust of Judah P. Benjamin, the Confederate secretary of war (later secretary of state). Benjamin sent Webster to deliver documents to secessionists in Baltimore, which Webster promptly passed on to Pinkerton and his staff. Webster was eventually arrested, tried as a spy, and sentenced to death. Though Lincoln sent President Jefferson Davis a message threatening to hang captured Confederate spies if Webster were executed, the death sentence was carried out in late April 1862.