The just war theory () is a doctrine, also referred to as a tradition, of military ethics that aims to ensure that a war is morally justifiable through a series of criteria, all of which must be met for a war to be considered just. It has been studied by military leaders, theologians, ethicists and policymakers. The criteria are split into two groups: ("right to go to war") and ("right conduct in war"). The first group of criteria concerns the morality of going to war, and the second group of criteria concerns the moral conduct within war. 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. The just war theory postulates the belief that war, while it is terrible but less so with the right conduct, is not always the worst option. Important responsibilities, undesirable outcomes, or preventable atrocities may justify war.
Opponents of the just war theory may either be inclined to a stricter pacifist standard (proposing that there has never been nor can there ever be a justifiable basis for war) or they may be inclined toward a more permissive nationalist standard (proposing that a war need only to serve a nation's interests to be justifiable). In many cases, philosophers state that individuals do not need to be plagued by a guilty conscience if they are required to fight. A few philosophers ennoble the virtues of the soldier while they also declare their apprehensions for war itself. A few, such as Rousseau, argue for insurrection against oppressive rule.
The historical aspect, or the "just war tradition", deals with the historical body of rules or agreements that have applied in various wars across the ages. The just war tradition also considers the writings of various philosophers and lawyers through history, and examines both their philosophical visions of war's ethical limits and whether their thoughts have contributed to the body of conventions that have evolved to guide war and warfare.
In the twenty-first century there has been significant debate between traditional just war theorists, who largely support the existing law of war and develop arguments to support it, and revisionists who reject many traditional assumptions, although not necessarily advocating a change in the law.
Origins
Ancient Egypt
A 2017 study found that the just war tradition can be traced as far back as to Ancient Egypt. Egyptian ethics of war usually centered on three main ideas, these including the cosmological role of Egypt, the pharaoh as a divine office and executor of the will of the gods, and the superiority of the Egyptian state and population over all other states and peoples. Egyptian political theology held that the pharaoh had the exclusive legitimacy in justly initiating a war, usually claimed to carry out the will of the gods. Senusret I, in the Twelfth Dynasty, claimed, "I was nursed to be a conqueror...his [Atum's] son and his protector, he gave me to conquer what he conquered." Later pharaohs also considered their sonship of the god Amun-Re as granting them absolute ability to declare war on the deity's behalf. Pharaohs often visited temples prior to initiating campaigns, where the pharaoh was believed to receive their commands of war from the deities. For example, Kamose claimed that "I went north because I was strong (enough) to attack the Asiatics through the command of Amon, the just of counsels." A stele erected by Thutmose III at the Temple of Amun at Karnak "provides an unequivocal statement of the pharaoh's divine mandate to wage war on his enemies." As the period of the New Kingdom progressed and Egypt heightened its territorial ambition, so did the invocation of just war aid the justification of these efforts. The universal principle of Maat, signifying order and justice, was central to the Egyptian notion of just war and its ability to guarantee Egypt virtually no limits on what it could take, do, or use to guarantee the ambitions of the state.
India
The Indian Hindu epic, the Mahabharata, offers the first written discussions of a "just war" (dharma-yuddha or "righteous war"). In it, one of five ruling brothers (Pandavas) asks if the suffering caused by war can ever be justified. A long discussion then ensues between the siblings, establishing criteria like proportionality (chariots cannot attack cavalry, only other chariots; no attacking people in distress), just means (no poisoned or barbed arrows), just cause (no attacking out of rage), and fair treatment of captives and the wounded.