War itself involves a great deal of diplomacy between those fighting, various allies of those fighting, neutral 3rd parties etc. Treatment of prisoners of war

War itself involves a great deal of diplomacy between those fighting, various allies of those fighting, neutral 3rd parties etc. Treatment of prisoners of war is probably the most obvious example of this. If you’re known to treat people who surrender well then you get more people willing to do so compared to a reputation as a country which will simply slaughter anyone captured. Sure it forces you to devote resources to caring for potentially millions of people, but that’s preferable to fighting millions of desperate people.