“There are such rules in war as all combatants agree to abide by, which is why even the Nazis fed and housed Allied P.O.W.’s in accordance with the Geneva Conventions of which Germany was a signatory.”
Not all our enemies have been so inclined, as we found out the hard way. Japan didn’t treat POWs like that and neither did the Koreans or the Vietnamese or the Muslims.
You are right. And we would be justified in returning the disfavor and ignoring the Geneva Convention when our enemies do not abide by it — especially when then neither abide by it nor are signatories to it.