I'm a bit unclear on the exact question, but it seems that you're asking when warfare became more humane vis-a-vis prisoners, wounded, and surrendered armies. I'll note that these values are not on a linear progression. The conduct of warfare varied in time and place. Conflicts long in the past could be more humane than warfare in the 20th century or even present. For example, in early modern Southeast Asia the population density was low. Warfare had a certain theatre element about it. Battles were short, sometimes settled through small proxy fights between individuals. The key was to conserve and capture population, then settle those people in new areas, thereby growing your state. It was far less destructive than the type of modern warfare seen in the world wars.