(For argument's sake, take "modern" to mean 1929 onwards, which was the year the Third Geneva Convention [relative to the Treatment of Prisoners of War] was introduced.)
Certainly the policies laid out in 1929 didn't come out of nowhere, and the 'modern' concept of Prisoner of War was one that had been evolving. What might interest you is the parole system used during the Civil War, which /u/anastik discussed in a question I actually asked a few years back.