Just warwar as being permissible according to a set of moral or legal rules. The rules applied may be ethical, religious, or formal (such as international law). The rules classically cover the justification for the war (Jus Ad Bellum) and the conduct of the participants in the war (Jus In Bello).
Just war theory has ancient roots. Cicero discussed this idea and its applications. Augustine of Hippo and Thomas Aquinas later codified a set of rules for a just war, which today still encompass the points commonly debated, with some modifications.
In modern language, these rules hold that to be just, a war must meet the following criteria before the use of force:
- War can only be waged for a just cause, such as self-defense against an armed attack.
- War can only be waged under legitimate authority. The sovereign power of the state is usually considered to be legitimate authority. This means that citizens at their own will cannot attack another country without the permission of the sovereign.
- War can only be waged with the right intention. Correcting a suffered wrong is considered a right intention, while material gain is not. Thus a war that would normally be just for all other reasons would be made unjust by a bad intention.
- War can only be waged with a reasonable chance of success. It is considered unjust to meaninglessly waste human life and economic resources if defeat is unavoidable.
- War can only be waged as a last resort. War is not just until all realistic options which were likely to right the wrong have been pursued.
- The force used must be proportional to the wrong endured, and to the possible good that may come.
- The acts of war should be directed towards the inflictors of the wrong, and not towards civilians caught in circumstances they did not create. Some theologians believe that this rule forbids weapons of mass destruction of any kind, for any reason (such as the use of an atomic bomb).
- Torture, of combatants or of non-combatants, is forbidden.
- Prisoners of war must be treated respectfully.