War parties usually don’t want to completely eradicate the opponent’s population, just break their fighting power and force them to surrender. The “tolerated” form of war is a power struggle between those who want power (incl. keeping it, so defending yourself), and it should leave out those who don’t as much as possible. So the idea is that you only fight the people on the other side who actually signed up for fighting, and spare those who would rather flee or accept defeat. Ofc in reality it’s never that clear cut, soldiers can be forced to fight against their will for example.