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]
(..) Bogs spent a week in the hole. Time's up, Bogs. It's your world, boss. Return to your cellblocks for evening count. All prisoners report for lock down. What? -Where's he going? -Grab his ankles. Help! Two things never happened again (..)1994 The Shawshank Redemption
(..) Skeets? Floyd! Four brave souls. Return to your cellblocks for evening count. All prisoners, return to your cellblocks. Turn to the right! Eyes front. This is Mr. Hadley. He's captain of the guards. (..)1994 The Shawshank Redemption