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]
(..) You fucking fascist! Did you see the film we made of the San Marino jail breakout... demonstrating the rising up of a seminal prisoner class infrastructure? You can blow the seminal prisoner class infrastructure out your ass! l'm not knocking down my goddamn distribution charges! Man, give her (..)1976 Network
(..) The first attempt on President Ford's life was days ago... and again yesterday in San Francisco. ln spite of the two attempts, Mr. Ford says... he will not become a prisoner of the Oval Office... a hostage of would-be assassins. The American people are good people. (..)1976 Network