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]
(..) Man, I had to. I had to. Oh, man, over there. Who is that stone-cold fox? Oh, it's my girlfriend. MAN: Take no prisoners, bro. MIKE: Oh, hey, there's my girl. - Hey. I'm glad you're here, Scar. This whole scout thing has me wicked nervous. I just feel like my whole future's (..)2009 17 Again