AHA! Philosophy Now! reprinted the article on Just War doctrine! Read it!
When is it right to go to war? How should war be waged? The Church in the Middle Ages developed a complex doctrine of the Just War, which is still of philosophical interest today. The source of many of these ideas was St Augustine, but his ideas were refined and extended by mediaeval philosophers and theologians, most famously in Thomas Aquinas’ essay De Bello. According to the Just War doctrine, waging war was justified if and only various conditions were met. The exact number and nature of those conditions varies from writer to writer although there is a great deal of overlap. Aquinas says there are three conditions for a just war:
1) Auctoritas (Just authority). Only the legitimate rulers of the state may declare war. [This is because people below the rank of sovereign can settle their disputes in the law-courts. It is only sovereigns who have no higher authority to arbitrate.]
2) Causa (Just cause). In general, nation X may wage war on nation Y only if Y has done some injury either to X or to X’s allies or friends. [It isn’t clear whether Y having harmed Y’s own people is also a just cause for X to wage war on it].
3) Intentio (Right intentions). The intentions of the warriors taking part must be the achievement of peace and of the just cause – not revenge, the desire for plunder or the suffering or destruction of the people on the other side.
Other thinkers such as Alexander of Hales, Christine de Pisan and Hugo Grotius gave additional conditions, notably:
4) Proportionality. The anticipated good must not be outweighed by the bad likely to be caused along the way.
5) Probability of success. There must be a reasonable prospect that the war will succeed.
6) Last resort. Peaceful alternatives must all have been exhausted first.
Later thinkers worried not only about when it was just to declare war, but also about how justly to conduct a war once it had started. The conditions for justly conducting wars were:
1) Proportionality (again). Acts of war must not be out of proportion to the provocation or the needs of the situation.
2) Discrimination. No killing of innocent civilians or of noncombatants such as medics and camp-followers.
The various philosophers all tended to agree on some, at least, of the central criteria for a Just War. Where they very often disagreed was over the casuistry – the application of these principles to actual cases. As can be seen from the list above, the conditions are open to widely varying interpretations.