Overview[]
Military necessity\necessity of war, along with distinction, and proportionality, are three important principles of international humanitarian law governing the legal use of force in an armed conflict. That is to say an attack or action must be intended to help in the military defeat of the enemy; it must be an attack on a military objective, and the harm caused to civilians or civilian property must be proportional and not "excessive in relation to the concrete and direct military advantage anticipated".
The Allied nation in World War II and the UN Forces in the Korean War tried to avoid killing civilians to the greatest extent possible at first, but in most cases they eventually chose to bomb cities believing it was militarily necessary.
War crimes are not allowed since they aren't necessary.