Distinctions

In any society, killing other people is forbidden (except in very special circumstances like self-defence or death penalty).
Since war is all about killing people, some distinction needs to be made between members of your own tribe and the enemy.
Sometimes, outsiders are recognizable by their physical appearance, like skin color and other racial features. Often, they speak a different language, and there are other cultural differences as well, like religion, rites and clothing.
Distinguishing the enemies from "us" is not enough, though. In order to justify killing, the other must be considered evil or sub-human.