Camp Douglas, in Chicago, Illinois, was one of the largest Union Army prisoner-of-war camps for Confederate soldiers taken prisoner during the American Civil War. Based south of the city on the prairie, it was also used as a training and detention camp for Union soldiers. The Union Army first used the camp in 1861 as an organizational and training camp for volunteer regiments. It became a prisoner-of-war camp in early 1862. Later in 1862 the Union Army again used Camp Douglas as a training camp.