He was captured by rebel forces and kept as their 'prisoner' for several months before they set him free.
They 'took him prisoner'. = He was 'taken prisoner'. [=they captured him and made him their prisoner]
a 'political prisoner' [=a person put in prison because of his or her political beliefs]
(..) And you give it all away to these starving scarecrows. I'd rather a plague of locusts around here. Don't scold me, Scarlett, please. I've just heard that Ashley was taken prisoner. -Ashley, a prisoner! -Yes. And maybe if he's alive and well he's on some Northern road right now. (..)1939 Gone with the Wind