Actually, the military teaches its soldiers and officers that war is won by making the other side surrender, not by being nasty. The greatest victory is one where they surrender without a single battle being waged (pretty sure that’s in the Art of War).

To that point, the enemy will only surrender if they know you will accept their surrender and treat them decently rather than torturing and/or killing them. War-crimes make it harder to win wars by convincing the enemy they have nothing to lose,