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]
(..) Yeah. We just ran focus groups at Ryan Correctional facility. We call this combat mode. It really put the fear of God into the prisoners. Yeah, that's okay. Get a load of this. What is that? He transforms. Kids love it. Really? (..)2014 RoboCop