For example, the Germans in WWII generally followed the established rules of war among European countries for the treatment of wounded and prisoners. However, the Japanese in WWII did not respect these conventions, and in response the U.S. gave less respect to Japanese wounded and prisoners than they did to Germans.
Uh, not on the Eastern Front, the Germans were murderous and even genocidal in their war against 'the Slavs' and the Russians returned the favour. The extreme racism in the Pacific contributed to a similar lack of restraint, see
With the Old Breed and Gore Vidal's memoirs of the war in the Pacific.
As to the OP there was a sense of honour and chivalry and Quarter in at least romantic versions of the Middle Ages was there not? I'm not talking historically accurate, after all we have elves and dragons too.