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 the two dead girls. MAN : All right, now, hold on! GUARD: Up on the tower! New prisoner's coming in! You men down in the yard, step off the fence! Clear that gate! Put me through to E Block. E Block. Yeah. Right. Oh. BRUTAL: Paul? Prisoner. Christ, give me a minute. You all right in there? For a man pissing razorblades, yeah. Ahh... You should've took the day off (..)1999 The Green Mile