When POWs were captured, they were either executed, exchanged, put into slavery or sometimes incorporated into the enemy army. I'm trying to find a few examples (3 would do) of the last category. Would their loyalty an issue in such a case?
During WW2 you had:
- Russian Liberation Army. These soldiers fought for the Germans and consisted out of former POWs.
- British Free Corps. These fought also on the side of the Germans and partialy consisted out of British POWs and volunteers.
- National Committee for Free Germany. While technically not an army, but you may consider their efforts an attempt to destabilize the enemy army. Essentially it was a group of communist and POWs that took efforts to bring their ideals to the German front.
In the American Civil War, both sides recruited from their POWs. Most entered US service and smaller number of Union POWs entered Confederate service. See https://en.wikipedia.org/wiki/Galvanized_Yankees
In the Russian Civil war of 1918-21, the incorporation of prisoners was so standard operation that some people changed sides many times. They had to choose from being shot or dying from starvation in the camp, or maybe remaining alive by "voluntary" incorporation. Such mass incorporations happened practically after every battle.