You follow the rules of war because it’s in your interests to do so. You treat POWs fairly, because you want your opponent to treat your own captured people fairly. You don’t pull a false surrender move, because that will come back to haunt you when you need to actually surrender. You don’t use chemical weapons, because using them would mean your opponent starts using them.

Granted, this tends to work better with peer powers. When there’s an imbalance, the larger power often gets to do whatever they want.