Confederate officers thought slaves were powerless and oblivious - they were dead wrong. Leaders in the South would openly discuss troop movements and battle plans and leave important documents right under their noses, without any fear they would comprehend and relay the information.

Who would they tell? They were just butlers, deckhands on a rebel sympathiser's steamboat, or field workers. But some weren't just slaves - they were also spies working undercover as Union intelligence officers. 'The chief source of information to the enemy,' General Robert E Lee, commander of the Confederate Army, said in May 1863, 'is through our negroes.'

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