Just War Theory and the Gospel: A Necessary Distinction

J. Neil Daniels

Introduction

In his magnum opus, the three-volume Systematic Theology, the great Princeton theologian Charles Hodge offered the following sobering evaluation of warfare:

It is conceded that war is one of the most dreadful evils that can be inflicted on a people; that it involves the destruction of property and life; that it demoralizes both the victors and the vanquished; that it visits thousands of non-combatants with all the miseries of poverty, widowhood, and orphanage; and that it tends to arrest the progress of society in everything that is good and desirable. . . . It is also conceded that the vast majority of the wars which have desolated the world have been unjustif i able in the sight of God and man. Nevertheless it does not follow from this that war in all cases is to be condemned. (Systematic Theology, 3:365)

In response to a recent discussion on X surrounding the Crusades and the question of Christian pacifism, I thought it necessary to clarify the gospel's relationship to violence and warfare. The gospel is first and foremost the message of God's reconciling work in Christ, calling sinners to repentance, faith, and reconciliation with Himself (2 Cor 5:18–19). Yet this reconciliation also carries profound implications for how Christians are to engage with a fallen world still marked by injustice and violence. The historical development of just war theory offers a sophisticated framework within Christian thought for discerning the legitimate use of force. This framework, however, stands in stark opposition to the motivations, methods, and outcomes of the Crusades, which represent a profound distortion of Christian teaching and ethics.

The Definition and Nature of Just War Theory

Just war theory is a rigorous philosophical and theological attempt to articulate when, if ever, it is morally permissible to engage in warfare. It seeks to navigate between two untenable extremes: pacifism, which categorically rejects all violence, and militarism, which celebrates war without moral restraint. At its foundation, just war theory insists that the use of force must be governed by stringent criteria of justice, charity, and proportionality.

Classically, just war theory is divided into two major components:

Jus ad bellum (the justice of going to war): the moral conditions that must be satisfied before entering into war (e.g., just cause, legitimate authority, right intention, last resort, probability of success, proportionality).

Jus in bello (justice in the conduct of war): the moral principles that must govern actions during warfare (e.g., discrimination between combatants and non-combatants, proportionality in the use of force).

In essence, just war theory demands that war be waged only as a necessary and strictly limited evil in defense of justice and the innocent—never as a means of conquest, vengeance, or personal or national aggrandizement.

The Historical and Philosophical Foundations

The intellectual roots of just war theory can be traced to classical philosophy, particularly in the writings of Cicero, but its most influential development occurs within Christian theology. Augustine of Hippo (354–430) provided the first significant theological formulation, arguing that while war is tragic and rooted in human sin, it may be necessary to curb greater evil. Augustine emphasized that the motive behind warfare must be love of neighbor and the maintenance of a just peace—never hatred or greed.

Later, Thomas Aquinas (1225–1274) refined Augustine's ideas in his Summa Theologicae (II-II, q. 40), specifying the three main conditions for a just war: legitimate authority, just cause, and right intention. His synthesis of Aristotelian ethics with Christian theology provided the foundation for centuries of reflection on warfare within the Western Christian tradition.

Hugo Grotius (1583–1645), the Dutch jurist and theologian, marked a watershed moment in just war theory with his masterwork De Jure Belli ac Pacis (On the Law of War and Peace). Writing amid the devastation of the Thirty Years' War, Grotius sought to establish universal principles of justice that would transcend sectarian divisions. His innovation lay in grounding just war theory not merely in divine revelation but in natural law accessible to all rational beings. Grotius meticulously systematized the conditions for just war and extended its application to international relations, effectively secularizing the tradition while preserving its moral rigor. His work represents the crucial bridge between medieval theological formulations and modern international humanitarian law.

The Biblical Foundations for Just War Principles

While the New Testament emphatically calls Christians to peace, humility, and love (e.g., Matt 5:9; Rom 12:18; Heb 12:14), it does not categorically prohibit all use of force in every circumstance. Government authorities are described as bearing the sword legitimately to punish evildoers (Rom 13:1–4). The centurions praised in the Gospels (e.g., Matt 8:5–13; Luke 7:1–10) are not condemned for their military service. Revelation portrays Christ as the righteous judge who will one day defeat evil with final authority (Rev 19:11–16).

Moreover, Scripture repeatedly affirms the crucial distinction between personal vengeance, which Christians are forbidden to pursue (Rom 12:19), and the legitimate exercise of justice by duly constituted authorities (Rom 13:4). These passages suggest that while Christians are personally called to forgiveness and peace, there remains a circumscribed role for properly ordered force to restrain evil in a fallen world.

Why the Crusades Violate Just War Principles

The Crusades (1096–1291) cannot be justified by appeal to just war theory. Though often characterized as defensive campaigns to protect pilgrims or reclaim sacred sites, the Crusades were deeply entangled with political ambition, economic exploitation, and ecclesiastical manipulation. They systematically failed to meet the criteria of legitimate authority, just cause, or right intention.

Legitimate Authority: While the Pope claimed spiritual authority, his assertion of temporal authority over vast military enterprises was deeply contested, even within Christendom itself. Grotius would later recognize this problem, arguing that legitimate authority must be coupled with transparent governance and accountability.

Just Cause: The Crusades frequently exceeded any plausible defensive justification, resulting in offensive campaigns against not only Muslim powers but also Jewish communities and even fellow Christians (as witnessed in the catastrophic sack of Constantinople during the Fourth Crusade).

Right Intention: The preaching of the Crusades was saturated with appeals to base motives: promises of land and wealth, vengeful bloodlust, and the theologically dubious assurance of automatic spiritual merit through indulgences granted to Crusaders.

Furthermore, the indiscriminate slaughter of civilians—most infamously during the First Crusade's capture of Jerusalem in 1099—flagrantly violated jus in bello principles of non-combatant immunity. In short, the Crusades represent a profound corruption of Christian teaching and a betrayal of the gospel's radical call to humility, repentance, and mercy.

Conclusion

The gospel is fundamentally the message of God's redemptive love and reconciliation through Christ—not a mandate for territorial conquest or religious coercion. While Scripture allows for the sober reality of just war under stringent conditions, the Crusader mentality—characterized by aggression, triumphalism, and brutality—cannot be reconciled with the biblical vision of righteousness and neighborly love. Christians must categorically reject any romanticizing of the Crusades and instead uphold a gospel-shaped ethic that vigorously pursues peace, justice, and unwavering fidelity to Christ in all dimensions of human experience. As Grotius himself recognized, true Christian witness in a violent world requires not holy war but holy restraint—the measured application of force governed always by justice, mercy, and the ultimate goal of a durable peace.

Bibliography on Just War Theory

I. Classical and Foundational Texts

Augustine of Hippo. The City of God. Translated by Henry Bettenson. London: Penguin Classics, 2003.

Essential for understanding the earliest Christian reflections on war, justice, and peace. Augustine lays the groundwork for Christian just war thinking by distinguishing between the City of God and the earthly city.

Thomas Aquinas. Summa Theologica. Translated by Fathers of the English Dominican Province. Westminster, MD: Christian Classics, 1981.

Aquinas offers the most systematic medieval articulation of just war principles, including conditions for just cause, legitimate authority, and right intention (II-II, q. 40).

Grotius, Hugo. The Rights of War and Peace. Edited by Richard Tuck. Indianapolis: Liberty Fund, 2005.

Grotius, often called the "father of international law," provides an early modern synthesis of Christian natural law and just war theory, emphasizing the regulation of conflict by universal moral principles.

II. Theological Studies

Bell, Daniel M., Jr. Just War as Christian Discipleship: Recentering the Tradition in the Church Rather Than the State. Grand Rapids: Brazos Press, 2009.

Bell contends that just war theory belongs within Christian discipleship, not secular politics. He critiques the co-option of just war theory by statecraft and calls for a return to its ecclesial roots.

O'Donovan, Oliver. The Just War Revisited. Cambridge: Cambridge University Press, 2003.

O'Donovan revisits the just war tradition from a theological-ethical standpoint, carefully analyzing its foundations and challenging its misuses in contemporary geopolitical settings.

Ramsey, Paul. The Just War: Force and Political Responsibility. Lanham, MD: Rowman & Littlefield, 2002.

Ramsey provides a major twentieth-century Protestant treatment of the moral challenges of war, arguing for the necessity of responsible force to preserve order and protect the innocent.

III. Historical Analyses

Helgeland, John. "Christians and the Roman Army from Marcus Aurelius to Constantine." Church History 43.2 (1974): 149–163.

Helgeland examines early Christian attitudes toward military service, providing critical background for understanding the shift from Christian pacifism to justifications for war.

Johnson, James Turner. Just War Tradition and the Restraint of War: A Moral and Historical Inquiry. Princeton: Princeton University Press, 1981.

Johnson traces the historical development of just war theory, demonstrating how Christian thought contributed to the moral limitation of violence throughout Western history.

Russell, Frederick H. The Just War in the Middle Ages. Cambridge: Cambridge University Press, 1975.

Russell analyzes medieval Christian thought on just war, offering a scholarly assessment of canonical and theological discussions from Augustine through Aquinas.

IV. Philosophical and Ethical Contributions

Biggar, Nigel. In Defence of War. Oxford: Oxford University Press, 2013.

Biggar provides a robust defense of the just war tradition in response to modern skepticism about the morality of warfare, arguing that, in a fallen world, the use of force can be both necessary and morally right.

Elshtain, Jean Bethke. Just War Against Terror: The Burden of American Power in a Violent World. New York: Basic Books, 2003.

Elshtain offers a provocative theological and ethical defense of military action against terrorism, illustrating how just war principles apply to contemporary threats.

Walzer, Michael. Just and Unjust Wars: A Moral Argument with Historical Illustrations. 5th ed. New York: Basic Books, 2015.

Though secular, Walzer’s influential work examines the morality of warfare through case studies, shaping much of contemporary Christian reflection on just war issues.

V. Contemporary Evaluations and Critiques

Cahill, Lisa Sowle. Love Your Enemies: Discipleship, Pacifism, and Just War Theory. Minneapolis: Fortress Press, 1994.

Cahill engages critically with both pacifist and just war traditions, offering a nuanced Catholic moral theological perspective that seeks to reconcile gospel nonviolence with political responsibility.

Corey, David D., and J. Daryl Charles. The Just War Tradition: An Introduction. Wilmington, DE: ISI Books, 2012.

An accessible and balanced survey of the just war tradition, tracing its historical roots and evaluating its ongoing relevance for Christian ethics today.

Patterson, Eric. Just American Wars: Ethical Dilemmas in U.S. Military History. New York: Routledge, 2019.

Patterson applies just war theory to specific episodes in American military history, offering both theological and practical insights into the ethics of armed conflict.

VI. Suggested Primary Sources

Catechism of the Catholic Church. Vatican City: Libreria Editrice Vaticana, 1993.

(See especially §§2307–2317.)

The Catechism provides the official Roman Catholic teaching on war, peace, and the conditions under which armed force is morally permissible.

The Westminster Confession of Faith. Glasgow: Free Presbyterian Publications, 2003.

(See Chapter 23, "Of the Civil Magistrate.")

The Confession articulates the Reformed Protestant perspective on civil authority and the justified use of force under lawful government.

Council of Trent. The Canons and Decrees of the Sacred and Oecumenical Council of Trent. Translated by J. Waterworth. London: Dolman, 1848. (Session XXV.)