Just war theory - the tradition of moral and legal reasoning that specifies the conditions under which armed conflict may be morally permissible - is one of the most durable and practically consequential legacies of biblical moral theology in Western political philosophy. Developed by Augustine from biblical materials, systematized by Aquinas, extended by Grotius into international law, and reformulated by contemporary philosophers including Michael Walzer, it remains the dominant ethical framework for analyzing the morality of war in Western culture and international law.
The Theory and Its Development
The tradition develops through several distinct phases. Augustine of Hippo (354-430 CE) is the foundational figure: in City of God (413 CE), Against Faustus, and letters to various military commanders, he developed the first systematic Christian account of when violence is morally permissible. Thomas Aquinas (1225-1274) in the Summa Theologica (II-II, Q. 40) systematized Augustine's criteria into the classic tripartite framework: just cause, right intention, and legitimate authority. Francisco de Vitoria (1486-1546) and Francisco Suárez (1548-1617) extended the tradition to cover the conquest of the Americas and the emerging system of international relations. Hugo Grotius (1583-1645), drawing on the biblical and natural law traditions, developed just war principles into the foundations of modern international law. Michael Walzer's Just and Unjust Wars (1977) reformulated the tradition in secular terms for contemporary political philosophy and international relations.
Biblical Texts Engaged
Romans 13:4 - 'for he is God's servant for your good. But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God's wrath on the wrongdoer' - is the foundational New Testament text for the authorization of coercive political power. Paul's account of the ruler as God's servant bearing the sword provides Augustine and his successors with the biblical warrant for the state's use of force in punishing evil-doers and protecting the innocent. The 'sword' is the instrument of justice in the hands of legitimate authority: it is not private vengeance but public justice acting on behalf of God's moral order.
Deuteronomy 20:10-13 - 'When you draw near to a city to fight against it, offer terms of peace to it. And if it responds to you peaceably and it opens to you, then all the people who are found in it shall do forced labor for you and shall serve you. But if it makes no peace with you, but makes war against you, then you shall besiege it' - provides, paradoxically, both authorization for warfare and significant limitation: the offer of terms of peace must precede military action, and proportionality in response to resistance is implied. Augustine and Aquinas both read Deuteronomy 20 as requiring that war be a last resort - preceded by exhausted efforts at peaceful resolution.
Matthew 5:9 - 'Blessed are the peacemakers, for they shall be called sons of God' - provides the eschatological horizon that complicates and qualifies just war theory. Augustine's crucial move is to argue that war, when genuinely just, is itself a form of peacemaking: the just war is fought not for conquest or vengeance but for the restoration of peace and the punishment of aggression. The just war tradition is therefore not a contradiction of the Gospel's peaceable vision but a recognition that, in a fallen world, the pursuit of peace may sometimes require the use of force. This is Augustine's answer to Manichaean pacifism and to those who argued that Christians could never participate in warfare.
Core Argument
The classical just war framework specifies two sets of criteria. The jus ad bellum (justice of going to war) criteria ask: Is there a just cause (typically: defense against aggression, protection of the innocent, punishment of grave injustice)? Is the intention right (restoring peace and justice, not conquest or vengeance)? Is the authority legitimate (the state or comparable authority, not private individuals or factions)? Is war a last resort (all reasonable alternatives exhausted)? Is there a reasonable prospect of success (avoiding futile violence)? Is the anticipated good proportionate to the anticipated harm?
The jus in bello (justice in the conduct of war) criteria ask: Are combatants distinguished from non-combatants (the principle of discrimination)? Is the harm inflicted on civilians proportionate to the military advantage gained (the principle of proportionality)? Are combatants using only permitted means of warfare (prohibitions against poison, treachery, and certain categories of weapons)?
These criteria derive from the biblical tradition's account of God as a just ruler who punishes evil, protects the innocent, and uses force proportionately and as a last resort. They have been progressively encoded in international humanitarian law, from the 1864 Geneva Convention to the Rome Statute of the International Criminal Court.
Intellectual Context
Augustine was responding to two challenges: the pacifism of the pre-Constantinian Church (which had generally prohibited Christian participation in warfare) and the Christian enthusiasm for imperial warfare after Constantine (which threatened to sacralize every Roman military enterprise). His just war theory steers between these extremes: not all warfare is unjust, but not all warfare is just; the moral analysis of each conflict is required.
Aquinas integrated Augustine's scattered reflections into a systematic framework using Aristotelian distinctions about causes, intentions, and proportionality. Grotius, in De Jure Belli ac Pacis (1625), developed the tradition into a comprehensive account of international law that grounded the just war criteria in both natural law and biblical reasoning, making them applicable to states of different religious traditions - a crucial step in the development of international humanitarian law.
Reception and Critique
John Howard Yoder and Stanley Hauerwas have argued from within the Christian tradition that just war theory represents a Constantinian compromise - an accommodation of the Church to imperial power - and that the Sermon on the Mount's ethic of nonviolence is the proper Christian political stance. This pacifist tradition continues to generate vigorous theological debate. From the other direction, realists in the Niebuhrian tradition argue that just war theory is insufficiently attentive to the tragic character of political choice and the irreducible role of power in international relations.
Legacy
Just war theory is the ethical foundation of international humanitarian law, the laws of armed conflict, the Geneva Conventions, and the International Criminal Court's jurisdiction over war crimes and crimes against humanity. The philosophical tradition that began with Augustine's reading of Romans 13 and Deuteronomy 20 has become the ethical framework within which military commanders, political leaders, and international lawyers make decisions about the use of force.
Key Passages
Augustine: 'The desire for harming, the cruelty of vengeance, the restless and implacable mind, the savageness of revolt, the lust of domination, and such like - these are the things which are to be blamed in war.' (Against Faustus, XXII.74)
Aquinas: 'For a war to be just, three things are necessary. First, the authority of the sovereign by whose command the war is to be waged... Second, a just cause is required... Third, it is necessary that the belligerents should have a rightful intention.' (Summa Theologica II-II, Q. 40, Art. 1)
Contemporary Relevance
The just war tradition remains the primary ethical framework for analyzing contemporary questions about humanitarian intervention, drone warfare, cyber warfare, and nuclear deterrence. Its criteria - particularly the requirements of discrimination, proportionality, and last resort - continue to generate vigorous philosophical debate and to provide moral resources for both the justification and the critique of specific military actions.