The just war tradition: Augustine, Aquinas and modern developments

Program / Course 22: Christian Ethics / Lesson 4.2

What you will learn

• State the main criteria of the just war tradition, divided into the right to go to war and right conduct in war.
• Describe the contributions of Augustine, Aquinas and later writers.
• Explain how the tradition is understood in Catholic, Orthodox and Protestant teaching.
• Summarize the main criticisms of the tradition and the replies to them.

The big idea

The just war tradition holds that war is a great evil, but that using armed force can sometimes be justified, and even required, to protect the innocent and restore justice. It sets conditions: war must be declared by lawful authority, for a just cause, with right intention, as a last resort, with a reasonable hope of success and in proportion to the wrong. It must also be fought justly, sparing noncombatants. The tradition is not a license to fight. Its purpose is to restrain war, and critics ask whether it does so.

Key terms

Jus ad bellum

Latin for right toward war. The conditions that must be met before resorting to force.

Jus in bello

Latin for right in war. The rules of just conduct once fighting has begun.

Just cause

A grave wrong that justifies force, such as defense against aggression or protection of the innocent.

Last resort

The condition that peaceful means have been tried or are clearly inadequate.

Proportionality

The requirement that the good to be achieved outweigh the harm caused, both in the decision to go to war and in particular actions.

Discrimination (noncombatant immunity)

The rule that civilians and others not taking part in fighting must not be directly and intentionally attacked.

Right intention

The requirement that war be fought to restore peace and justice, not for vengeance, domination or cruelty.

Jus post bellum

A more recent addition: justice after war, including fair settlement and reconstruction.

Background

Cicero and other Roman writers had spoken of just wars, with declared causes and lawful conduct. Ambrose of Milan, a bishop and former governor, applied such ideas to Christians in the late fourth century, arguing that defending the weak against violence is a duty of justice. His pupil Augustine gave the approach its enduring form, writing in the years after the sack of Rome in 410 and in the face of invasions across North Africa.

Augustine was not a pacifist, but he held that war is a tragic necessity in a fallen world. In Against Faustus he argued that what is evil in war is the love of violence, the lust for power and the cruelty, not the use of force by lawful authority to restore peace. In his letter to Boniface, a Roman commander, he wrote that peace is the aim of war and that one must be a peacemaker even in fighting. In City of God, Book 19, he describes peace as the tranquility of order and describes war as sometimes waged by the just to end greater injustice. His texts are scattered and occasional, and later writers gathered them into a system.

Step-by-step explanation

Step 1: Aquinas and the three conditions

Aquinas, in the Summa Theologiae (Second Part of the Second Part, Question 40), asked whether it is always sinful to wage war. Following Augustine, he answered that it is not, and he set out three requirements. First, the authority of the ruler, since private persons may not declare war and a ruler has the care of the common good. Second, a just cause: those attacked must deserve it on account of some wrong. Third, a right intention, that the aim be the advancement of good or the avoidance of evil, and not cruelty, revenge or the lust for domination.

He also discussed related questions, such as whether clergy may fight, which he denied, and whether stratagems are lawful. His account was brief, and it became the core of later discussion. Francisco de Vitoria and Francisco Suarez, Spanish theologians of the sixteenth century, extended it to international law and to the rights of Indigenous peoples. Hugo Grotius, a Protestant, developed a law of nations in the seventeenth century in which similar ideas appear in a more secular form.

Step 2: The classic criteria

Over time, the tradition expanded the conditions for going to war. Besides legitimate authority, just cause and right intention, writers add last resort, a reasonable probability of success, and proportionality of the expected good to the expected evil. For conduct in war, two rules are central: discrimination, which forbids directly targeting noncombatants, and proportionality, which forbids harm out of proportion to the military good sought. Modern writers also add rules about prisoners and the treatment of the wounded, which international humanitarian law, such as the Geneva Conventions, now enforces.

These criteria are to be applied together. A war that fails one of them is unjust. The tradition holds that the burden of proof lies against war, a presumption against the use of force, and in its modern Catholic form this presumption is stated explicitly. The criteria are tests that require judgment, and honest observers can disagree about whether they are met in a given case, such as the invasion of a country to stop atrocities or a preventive strike against a feared threat.

Step 3: Teaching in the traditions

The Catholic Church teaches the tradition in the Catechism (paragraphs 2308 to 2310), which lists strict conditions for legitimate defense by military force: the damage must be lasting, grave and certain, all other means must have been shown impractical, there must be serious prospects of success, and the use of arms must not produce evils graver than the evil to be eliminated. Gaudium et Spes of the Second Vatican Council (1965) condemned acts of war aimed at indiscriminate destruction of cities as a crime against God and man. In 1983 the American bishops’ pastoral letter, The Challenge of Peace, applied the criteria to nuclear weapons. Popes in recent decades have increasingly questioned whether modern war can be just, and Pope Francis in 2020 wrote that it is hard to think of a just war in the present age. The church has not repealed the doctrine, and theologians debate how far it has shifted.

Protestant Reformers accepted just war. Luther, in a 1526 treatise titled Whether Soldiers, Too, Can Be Saved, argued that the office of the sword is a divine ordinance and that a soldier may serve in good conscience in a just war. Calvin in the Institutes (Book 4, chapter 20) allowed lawful war by magistrates for defense and justice. The Augsburg Confession (Article 16) and the Westminster Confession (chapter 23) include the lawfulness of just wars. In the twentieth century, Protestant thinkers such as Reinhold Niebuhr, who offered a realist defense of force, Paul Ramsey and Oliver O’Donovan revived the tradition, each in different ways.

Orthodox churches have not developed a formal just war doctrine. They honor soldier saints and teach that defense of neighbors may be necessary, yet they hold that war is always tied to sin and that a soldier who kills needs repentance. Orthodox statements, such as the Russian Orthodox Church’s Basis of the Social Concept (2000), speak of war as an evil, allow participation in defensive war and condemn aggression. How Orthodox churches apply this has varied in practice.

Step 4: Criticisms and replies

Critics raise several objections. Pacifists say that Jesus’ teaching of love of enemies cannot be reconciled with killing, and that the tradition arose after the church allied with the empire. Realists, in a different way, say that states act on interest and that moral criteria are rationalizations. Some critics note that nearly every state claims its wars are just, and that the criteria are too elastic to constrain anyone. Others say that modern weapons, such as nuclear arms and mass bombing, make discrimination and proportionality impossible to satisfy.

Defenders reply that the tradition is meant as a standard to judge rulers, and that its presence has in fact limited war, as in the formation of laws against attacks on civilians. They argue that refusing to use force can leave the innocent unprotected, as in cases of genocide, and that love of neighbor may require defense of the neighbor. They also say that abuse of a standard does not show the standard to be false. The next lessons place this tradition beside pacifism and ask what each side can learn from the other.

1. Is the authority that declares the war legitimate, and does it act for the common good?
2. Is the cause just and grave, and is it the real cause or a pretext?
3. Have peaceful means been tried, and is there a reasonable chance of success?
4. Will the harm done be proportionate to the good achieved?
5. Are noncombatants protected, and are the means used permissible whatever the aim?
6. What would a pacifist reply to this case, and is the reply answered?

Examples

Different perspectives

Catholic

Catholic teaching holds that governments have the right and duty of legitimate defense, that war is always an evil, and that strict conditions apply. Recent popes have emphasized the presumption against force, called for disarmament and urged diplomacy. Catholic moral theologians continue to apply the criteria to cases such as nuclear deterrence, humanitarian intervention and drones, and they differ in their conclusions.

Orthodox

Orthodox teaching treats war as an evil and a consequence of the fall, allows the defense of neighbors, honors soldier saints and calls for repentance and care for those who have killed. It has no formal just war theory. Orthodox thinkers differ on how far to adopt Western criteria, and some stress the church’s peacemaking witness over its acceptance of force.

Protestant

The Lutheran, Reformed and Anglican confessions affirm just war. Contemporary Protestant ethicists, from Niebuhr to Ramsey and O’Donovan, have defended and refined it, while mainline church bodies have often issued statements stressing peacemaking and warning against war. Evangelicals hold a range of views, with many supporting just war and some pacifism.

Secular and scholarly

Secular philosophers such as Michael Walzer, in Just and Unjust Wars, have defended a secular version of the tradition and shaped international law and military ethics. Other scholars criticize it from realist, pacifist or legal perspectives. International humanitarian law, such as the Geneva Conventions, now binds states in conduct of war, though enforcement is imperfect.

Why it matters

Decisions about war are among the gravest a society makes, and Christians have to judge them as citizens, voters and sometimes as soldiers. The just war tradition gives a structured way of thinking that does not assume that all force is sinful or that all war is permitted. It is a skill of judgment that applies well beyond war, and it also gives a way to criticize leaders who claim a just cause too easily.

Common misunderstandings

Knowledge check

Think it through

1. Is there any war you think clearly met the criteria? Any that clearly did not? What are your reasons?
2. How might a Christian soldier apply the rules of conduct in a hard situation?
3. Does a presumption against force seem to you to be strong enough, too strong or about right?
4. What would a pacifist say to the argument that the innocent must sometimes be defended by force?

Independent assignment

Testing a war against the criteria

In your notebook, write 400 to 600 words. Choose one historical conflict that you can study from at least two sources. Apply each of the just war criteria to it, saying whether it was met and giving evidence. Identify the point on which reasonable people would most disagree, and state what a pacifist would say about the case. Do not send this to anyone or post it anywhere.

Further study

• Augustine of Hippo, The City of God Read Book 19, especially the chapters on peace and on the justice of war.
• Thomas Aquinas, Summa Theologiae Read Question 40 of the Second Part of the Second Part, on war.
• John Calvin, Institutes of the Christian Religion Read Book 4, chapter 20, on civil government and war.
• Catholic Church, Catechism of the Catholic Church Read the paragraphs on legitimate defense and the avoidance of war.
• Romans 13:4 The ruler bears not the sword in vain.
• Luke 3:14 Soldiers told to do violence to no man.
• Psalm 82:4 Deliver the poor and needy.
• Matthew 5:9 Blessed are the peacemakers.