Beginning with the great church father Augustine (354-430 AD.), Christian thinkers have developed criteria for distinguishing justified wars from unjustified wars. What they really tell us is which hard judgments we need to make. First come criteria for when going to war is permissible. It isn’t enough to honor most of them; all seven must be satisfied.

1. Public authority. War must be declared by a legitimate government. Private individuals and groups cannot do it.
2. Just cause. War must not be waged except to protect innocent life, to ensure that people can live decently, and to secure their natural rights.
3. Right intention. Not only must there be just cause to take up arms; this just cause must be the reason for taking up arms. Our goal must be to achieve a just peace.
4. Comparative Justice. War should not waged unless the evils that are fought are grave enough to justify killing.
5. Proportionality. There must be reason to expect that going to war will end more evil than it causes — this means not only physical evil, but spiritual — not only destruction of bodies and buildings, but corruption of callings and virtues.
6. Probability of Success. There must be a reasonable likelihood that the war will achieve its aims.
7. Last resort. War should not be waged unless a reasonable person would recognize that the peaceful alternatives have been exhausted.

Next come criteria for how war must be fought. No exceptions are allowed, no matter how much we may want to make them.

1. Right intention. Remember, the goal must be to achieve a just peace. Therefore, we must avoid any act or demand that would make it more difficult for our enemies to reconcile with us some day.
2. Proportionality. We must never use tactics that can be expected to bring about more evil than good.
3. Discrimination. Even though harm l might come to them accidentally, directly intended attacks on noncombatants and nonmilitary targets are never permissible.