Just War theory is a doctrine, also referred to as a tradition, of military ethics studied by theologians, ethicists, policy makers and military leaders. The purpose of the doctrine is to ensure war is morally justifiable through a series of criteria, all of which must be met for a war to be considered just. The criteria are split into two groups: ‘the right to go to war’ and ‘right conduct in war’ . The first concerns the morality of going to war and the second with moral conduct within war. Recently there have been calls for the inclusion of a third category of just war theory - jus post bellum - dealing with the morality of post-war settlement and reconstruction.
Just War theory postulates that war, while very terrible, is not always the worst option. There may be responsibilities so important, atrocities which can be prevented or outcomes so undesirable they justify war.
Origins
The Indian epic, the Mahabharata, offers one of the first written discussions of a 'just war'. In it, one of five ruling brothers asks if the suffering caused by war can ever be justified, and then a long discussion ensues between the siblings, establishing criteria like proportionality, just means, just cause, and fair treatment of captives and the wounded.
The war in Mahabharata is preceded by context that develops the "just cause" for the war including last minute efforts to reconcile differences to avoid war. At the beginning of the war, there is the discussion of "just conduct" appropriate to the context of war.
In ancient Rome, a "just cause" for war might include the necessity of repelling an invasion, or retaliation for pillaging or a breach of treaty. War was always potentially nefas, "wrong, forbidden", and risked religious pollution and divine disfavor. A just war thus required a ritualized declaration by the fetial priests. More broadly, conventions of war and