Basic Principles of Just War Theory

            Just war theory is traditionally divided into two parts: "jus ad bellum" which defines when it is justified to resort to armed force; and "jus in bello" which defines how armed force may be legitimately employed.

Principles of jus ad bellum:

1.  "Legitimate Authority" -- war must be declared and supervised by competent authorities, not by private or self appointed persons, and not by public officials exceeding their authority. Traditionally, the only competent authority is a sovereign – someone who cannot appeal to any higher authority. 

2.  "Just Cause" -- war is permissible only if it is pursued in order to right or prevent a serious wrong.  Self-defense is one clear case, but older writers listed other "just causes": regaining stolen property, and punishing evildoers. 

3.  "Last Resort" -- war is justifiable only when all peaceful or non-violent means to redress or avoid the wrong have failed.

4.  "Probability of Success" -- war is justifiable only when there is a reasonable belief that one is likely to win, otherwise the death and destruction is for nothing. 

5.  "Proportionality of ends" -- the good which one expects the war to accomplish must outweigh the death and destruction it will produce, including that inflicted on one's enemies.

6.  "Right Intention" -- one's 'just cause' must be the real reason for waging war, not a pretext or excuse for the pursuit of other goals.  One must aim at a just peace, not revenge or retaliation.

Principles of jus in bello:

1.  "Discrimination" or "Noncombatant Immunity" -- one must fight in a way that minimizes harm to noncombatants.  Certainly one must not directly target civilians, but also one should try to reduce the risk of harming them indirectly, even when this means increasing the risk to one’s own military personnel.

2.  "Proportionality of means" -- the particular tactics or means of fighting one uses must cause no more death and destruction than  is necessary to accomplish one's legitimate military objectives.  Even in the context of a just war, for example, one should not blow up an entire army with a nuclear bomb, if one could have induced that army to surrender by killing some smaller number.