Just war theory is a normative theory of international politics that engages with how war should be waged in order to maintain justice in state-to-state relations. This is different from a descriptive theory, like realism, that explains how the world is and how war occurs in the international system. The theory of just war is a part of the greater just war tradition.
The just war tradition is a synthesis of Western moral thought on war: its contributors include theologians, statesmen, philosophers, and soldiers. The canon usually names Augustine as the father of the tradition, as he was the first to interrogate whether it is ever morally correct for a Christian to go to war. However, the tradition began during the Pax Romana, when the statesman Cicero first developed a legal theory that required an enemy to have prior guilt to justify a war against them. Augustine, and later theologian Thomas Aquinas, built on ad bellum considerations (whether it is morally permissible to go to war).
Later Christian thought involved in bello considerations (principles during war that limit violence and unjust action). Modern secular thinkers, like Michael Walzer in Just and Unjust Wars, have widened the tradition to encompass more variations in thought and claims to justice. There have been multiple resurgences of the tradition in the twentieth and twenty-first centuries as thinkers reckoned with changes to the nature of war, including the nuclear revolution, increases in non-state terrorism, and a potential future of war fought with completely autonomous technology and weapons. Such shifts in the nature of war make claims of justice consistently important within the international political system and the international legal order, especially as states attempt to facilitate peace after violence ends.
When applying the early theory to violent outbursts during the fall of the Roman Empire, Augustine argued that all citizens, and especially Christians, should seek harmony and peace in society. This is due to caritas, or charity – Augustine’s motivating virtue for Christian participation in war. The Christian love of neighbor permits the use of force for defensive purposes, but love of enemy requires limits on the use of force needed to protect the neighbor. In Christian moral theology, all individual violence committed against human life entails misery of the self, and even justified violence should be exercised with a woeful spirit. Because of Christian virtue, participation in war is only justified when there are:
just intentions to enter war,
a just cause or vindication of justice,
just dispositions,
just auspices or proper authority to legitimate entering a war,
just conduct of war.
These early criteria are still present in some shape or form in the modern conception of just war theory.
What is Just War Theory?
The theory today has expanded to encompass not just an individual Christian’s participation in war, but whether the war is justified in its own right. There are three main components to just war theory that each have their own criteria. The components are jus ad bellum, justice in starting the war; jus in bello, or just conduct during a war; and jus post bellum, justice after the war (more generally, creating a sustainable and just peace after the war). The undergirding assumption of the theory is that the taking of human life is always murder unless the confrontation conforms to the principles of justice. In this thought, disproportionate or excessive use of force is morally wrong, and war is only justifiable for defensive purposes by a state.
The jus ad bellum principles that are prerequisites for resorting to war are:
just cause (self-defense or in defense of another state),
right/legitimate authority (a sovereign state),
right intention (eventual peace),
a public declaration of war,
war as a last resort (have all diplomatic and legal measures been exhausted?),
reasonable hope/chance of success.
Proportionality as a criterion fits both jus ad bellum considerations and in bello principles.
The jus in bello principles are:
proportionality (does the response fit the cause?),
discrimination/non-combatant immunity (those who do not contribute directly to the fighting of the war should be immune to direct attack).
Though more commonly associated with St. Augustine, Just War Theory traces its roots to Cicero.
These considerations are used to determine just means of waging war, usually by limiting military force. The just war tradition has faced challenges from power politics and normative restraints since its origin, especially when it comes to using moral or ethical considerations to potentially limit military effectiveness in a war. An example of unrestrained military necessity arguments include Germany’s Kriegsraison during World War II; a doctrine that rendered ordinary protection laws inoperative due to military necessity, ultimately leading to the systemic extermination of Jews, Poles, political and social dissidents, and other war-time atrocities. Jus in bello thinking is critical to the idea that the expediency and convenience of powerful states’ military choices must be moderated to prohibit atrocities.
Jus post bellum considerations are largely a reflection of jus ad bellum principles, as a justifiable war also implies certain rights and obligations for the belligerent state(s) when considering final outcomes. Post bellum considerations are also linked to the in bello component; as proportionality and noncombatant immunity criteria require restraint in combat, so too must total war and total conquest be suspect as a claim to justice.
Overall, there is general consensus from scholars on the formal components and criteria of just war, but there is significant debate over the substantive proscriptions and interpretations of the criteria themselves. For example, how do we identify non-combatants in modern war when combatants cannot be visually distinguished? This can lead theorists and decision-makers to dismiss justice considerations too easily during wartime deliberations.
Just War and Realism
Structural realism is a descriptive theory that explains how the world is, whereas just war is a normative theory that is concerned with how the world should be, specifically in terms of the justice of any given war and the actions of the states that undertake them. Realism and modern just war thinking share similar assumptions in their theories: the state is central to the international system,[1] there is an undeniable inevitability of war,[2] and because a state is motivated by survival in an anarchic international structure,[3] a state has a right to self-defense. The main distinction in the logics of the two theories is that Walzer justifies self-defense in moral terms. Structural realism treats state survival as an objective motivator, whereas just war defends concern with state survival through normative terms: it is not just necessary but morally good to enact policy that concerns itself with the survival of the political life within the territorial state. Because of this concern for justice and the common good, just war theorists have said it is permissible, if not required, for other states to intervene on behalf of oppressed groups. When applying these two theories to international politics, realist foreign policy can lead a Great Power to act with restraint in the system, whereas a justice oriented strategy could lead a Great Power to frequently intervene on behalf of other states.
[1] Walzer, Just and Unjust Wars, 2000:53
[2] Augustine, 1984:6, “God’s Providence constantly uses war to correct and chasten the corrupt morals of mankind, as it also uses such afflictions to train men in a righteous and laudable way of life.”
Originally published by Madeline Kasper at ondisc.nd.edu on January 05, 2026.