Rose O'Neal Greenhow was a famous Confederate spy during the American Civil War. A socialite in Washington, D.C., during the period before the war, she moved in important political circles and cultivated friendships with presidents, generals, senators, and high-ranking military officers including John C. Calhoun and James Buchanan. She used her connections to pass along key military information to the Confederacy at the start of the war. In early 1861, she was given control of a pro-Southern spy network in Washington, D.C., by her handler, Thomas Jordan, then a captain in the Confederate Army. She was credited by Jefferson Davis, the Confederate president, with ensuring the South's victory at the First Battle of Bull Run in late July 1861.

Tactical or battlefield intelligence became vital to both sides in the field during the American Civil War. Units of spies and scouts reported directly to the commanders of armies in the field, providing 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, he was often sentenced 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.