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]
(..) Deactivate. Failing the radar shield, the slightest release of pressure in the weight of the diamond... and the would-be thief is a prisoner. A prisoner of the Pink Panther. Why is it called the Pink Panther? The stone is flawed. (..)1975 The Return of the Pink Panther