The Catholic Church has long upheld a doctrine of Just War, providing moral criteria by which the use of force by nations may be judged. Rooted in Sacred Scripture, developed by St. Augustine, clarified by the Scholastics – especially St. Thomas Aquinas – and codified in authoritative theological manuals and catechisms, Just War Theory is not a license for militarism but a safeguard for peace and justice.
In an age of modern warfare, drone strikes, and nuclear deterrence, this venerable teaching remains both timely and underappreciated. In fact, a just war is one of five exemptions to the prescript “Thou shalt not kill.”
Foundations of Just War in Sacred Scripture
The Old Testament contains numerous examples of divinely sanctioned warfare. In Exodus 15:3, God is described as “a man of war,” affirming that war, under certain conditions, is not inherently evil. Judges and kings of Israel often led battles under divine mandate, as seen in the campaigns of Joshua (cf. Joshua 6) and David (cf. 2 Kings 5:19). Judas Machabeus led a successful war for freedom against the Seleucid Antiochus Epiphanes, a type of antichrist. God’s people suffered many martyrs (see 2 Machabees 6 & 7) but they also had angels fighting on their side (see 2 Machabees 3:24-26; 5:2-4).
In the New Testament, while our Lord Jesus Christ teaches peace and love of enemies (cf. Matthew 5:44), He does not condemn soldiers for their profession. When the centurion approaches Jesus in Matthew 8:5-13, Our Lord praises his faith without requiring him to abandon his role. Likewise, St. John the Baptist instructs soldiers: “Do violence to no man… and be content with your pay” (Luke 3:14), rather than telling them to leave military service.
Church history is littered with saints who also served as soldiers; notable among them are as St. Sebastian, St. George, Bl. Charlemagne, St. Ferdinand III, and most famously, St. Joan of Arc.
These scriptural passages and saintly exemplars illustrate a foundational principle: War is not intrinsically immoral, but its morality depends on context, authority, and intention.
St. Thomas Aquinas and the Classical Formulation
St. Thomas Aquinas systematized the Church’s teaching on Just War in Summa Theologiae, II-II, q. 40, De bello. He laid out three essential conditions for a war to be just:
- Legitimate Authority (auctoritas principis): War must be declared by a lawful sovereign, not by private individuals.
- Just Cause (causa iusta): There must be a real and grave reason, such as defense against aggression or rectification of grave injustice.
- Right Intention (intentio recta): The goal must be to advance good or avoid evil, not vengeance or conquest.
The Angelic Doctor emphasizes that even with these conditions, war remains a sad necessity, not a virtue. The aim is peace, not perpetual conflict.
The Catechism of the Council of Trent on War
The Roman Catechism, or Catechism of the Council of Trent (1566), reflects St. Thomas’ scholastic principles and applies them pastorally. In the section on the Fifth Commandment, it teaches:
“The killing of a man by public authority is not murder, but a lawful act of justice… Those who bear the sword wield it as the ministers of God” (Catechism of the Council of Trent, Part III, Commandment 5).
It acknowledges that rulers may justly wage war for defense and the punishment of evildoers. Yet it warns against wars of ambition or revenge, reinforcing the moral gravity of such decisions.
Insights from Traditional Theological Manuals
Fr. Dominic Prummer, in his Manuale Theologiae Moralis, offers a succinct but authoritative summary of Just War doctrine. He reiterates St. Thomas Aquinas’ three conditions and adds further criteria, such as:
- Proportionality: The expected good must outweigh the harm.
- Last Resort: All peaceful alternatives must be exhausted first.
- Reasonable Hope of Success: War must not be futile.
These manuals – once standard in seminaries – serve as a bridge between scholastic theory and pastoral application. Their clarity contrasts sharply with many modern reinterpretations which have allowed liberalism and false ideas to attack the structured and systematic theology that was once widely taught in all seminaries before Vatican II.
Application to Contemporary Conflicts
Applying traditional Just War Theory to modern conflicts reveals both its enduring relevance and the difficulty of satisfying its conditions.
(1) Legal War
To be just, a war must be declared by the lawful authority. Naturally this precludes any surprise attack because a state of war must first be declared.
In the United States, the Constitution is the highest law of the land; it is the legal ‘contract’ which created this modern nation. The Constitution clearly states that Congress has the rightful authority to declare war (Article 1, Section 8). This power does not reside in the executive or legislative branches of the U.S. government. Many have argued this was a wise decision because it is far easier for one man (or a small group) to desire and advance an unjust war than for a larger elected body of officials. Congress declaring war is no guarantee that a war is just, but it does increase that probability.
It is a sad part of U.S. history that this nation has been involved in wars for nearly the entirety of the past six decades. Nevertheless, the last time Congress declared war was in 1942, following the attack on Pearl Harbor during World War II. As Catholics we do well to realize that our nation, and the world at large, will not have peace until we obey Our Lady of Fatima.
(2) Preemptive War
Many 21st-century conflicts – such as the U.S. invasion of Iraq in 2003 – have been justified to the world as preemptive measures. However, traditional teaching does not allow for war based on speculative threats: “A preventive war, to forestall an uncertain future aggression, lacks just cause according to Thomistic principles” (Summa Theologiae, II-II, q. 40; cf. Prummer, n. 507).
(3) “Total War” and Civilian Casualties
Modern weaponry and the loss of Christian restraint greatly complicates the principle of proportionality. In feudal centuries past, battles were waged in hand-to-hand combat, warfare was suspended for liturgical and agricultural seasons, and women and children were never targeted.
The twentieth century, however, has seen the concept of “total war” become the dominant method of warfare. An “unconditional surrender” was unheard of in centuries past, but modern weaponry has made it possible for victors to force this dishonor upon a defeated enemy.
The first “total war” in world history, where targeting innocents was part of the strategy for victory as well as destroying farmland and social infrastructure en masse, was waged by the North upon the South in the United States’ Civil War (1861-1865). Not even the terrible Napoleonic Wars that ravaged Europe were this barbaric. Yet the two World Wars both followed the horrific path of total war and unconditional surrender.
(4) Drone Warfare and Civilian Casualties
Modern technology also severely restricts the possibility of a just war. Civilian deaths – including non-combatants, the aged, the infirm, women, and children – from drone strikes or bombing campaigns challenge the notion that war is being waged with rightful intention and proportionate means: “Thou shalt not kill the innocent and just” (Exodus 23:7). The direct killing of non-combatants cannot be justified under the doctrine of Double Effect.[1] Such murder can never be morally justified when it is intended as part of the military strategy to ‘demoralize a population’ and pummel an opponent into submission so that a great percentage of its innocent population is not killed.
(5) Nuclear Weapons
The Church has long expressed grave concern over weapons of mass destruction. However, according to traditional Catholic moral theology, these weapons are not intrinsically immoral by their nature but must be judged by the manner and circumstances of their use. Pope Pius XII, in his September 30, 1954 address to the World Medical Association, clarified that “one cannot even in principle raise the question of the lawfulness of atomic, chemical, and bacteriological warfare, except in the case where it must be considered essential to defend oneself under the conditions indicated.” He further warned that when the destructive effects of such weapons extend beyond human control – causing indiscriminate annihilation of civilian life – “their use must be rejected as immoral.”
The possession of nuclear weapons, when oriented strictly toward deterrence and self-defense, may be morally tolerable, provided it does not entail an intention to commit unjust or disproportionate acts. Fr. Bernard de Lacoste notes that “military nuclear power is not in itself immoral,” though its moral use is extremely rare and requires the most stringent conditions: the target must be clearly military, the death of civilians must be unintended and limited, and the expected military advantage must be proportionate to any collateral harm. These principles are rooted in the Thomistic doctrine of Double Effect (Summa Theologiae, II-II, q. 64, a. 6-7) and the perennial teaching that one may never do evil so “that there may come good” (Romans 3:8).
A nuclear first strike, or even a retaliatory one involving widespread civilian casualties, is therefore virtually impossible to reconcile with traditional Catholic criteria for a just war. While possession for the sake of deterrence may be tolerated under certain conditions, the actual use of such weapons remains morally permissible only in the most narrowly defined and practically unlikely circumstances.
A Restatement of Principles for Today
The confusion in post-conciliar moral theology on Just War Theory often stems from a departure from the Thomistic foundation. The traditional teaching provides a clear set of objective criteria, an insistence on moral intention over political expediency, and a commitment to peace as the final goal.
Just War Theory, in its traditional Catholic formulation, is a solemn and serious moral doctrine – not a loophole for political violence. Rooted in Scripture, deepened by the Scholastics, and codified by the Church’s authoritative texts, it calls rulers and citizens alike to pursue justice and peace with gravity and discernment.
In a world increasingly prone to warfare by executive decree and high-tech annihilation, this doctrine is more necessary than ever. The faithful must pray, study, and advocate for a return to the clear, uncompromising standards of Catholic tradition on matters of war and peace. And likewise, we must ensure that sound theology is taught in the seminaries, preached to the faithful, and codified in the catechisms of today and tomorrow.
Pray the Rosary for Peace
There is nothing more powerful we can do than pray the Rosary daily for peace. Our Lady Herself has instructed us thus at Fatima.
I want you … to continue praying the Rosary every day in honor of Our Lady of the Rosary, in order to obtain peace in the world and the end of the war, because only She can help you.”
She also told us that if we do not cease offending God, convert, pray and sacrifice, then God will
“…punish the world for its sins, by means of war, famine, and persecutions against the Church and of the Holy Father.”
These prophecies are being fulfilled in our time. Thus, every Catholic has a serious obligation to live the Message of Fatima, not only for the salvation of their own soul, but for the good of the world at large. As She Herself has said, at this point in history, only She can help us.
ENDNOTE:
[1] Double Effect is the philosophical principle based on the idea that there is a morally relevant difference between an “intended” consequence of an act and an “unintended” consequence, one that is foreseen but not expected to occur.