In the film "Saving Private Ryan," didn't our soldiers shoot a couple of surrendering Germans after we secured Omaha Beach? I bet that happended a lot in the immediate aftermath of a battle, and that few if any soldiers were court-martialed for it. I bet POW protection only really kicks in after the dust has settled, after the POW's have been formally processed and incarcerated.
Remember Dunkirk? The Nazi's butchered many British POW's. When the British got their retribution (in smaller degrees) the press at that time at least turned a blind eye.