The question of when, if ever, it is morally permissible to go to war has been debated by Christians for nearly two millennia. 

The roots of Just War theory trace back to St. Augustine, who occasionally discussed the justice or injustice of war in writings such as Against Faustus. Augustine did not create a systematic theory, but his scattered reflections laid the groundwork for later thinkers. 

By the 12th century, the canonist Gratian, in his Decretum (c. 1140), cited Augustine dozens of times in discussing war, showing how the Church had begun to treat military action as a moral question, not just a political or practical one.

The first clear articulation of a systematic Just War theory came from St. Thomas Aquinas. In his Summa Theologiae (II-II, 40, 1), Aquinas wrote: “In order for a war to be just, three things are necessary: proper authority, just cause, and right intention.” 

Each of these criteria builds on Augustine’s scattered comments. Proper authority means only a lawful government—not private citizens—can declare war: “It is not the business of a private individual to summon together the people” (ST II-II, 40,1). 

Just cause historically meant responding to aggression or correcting a grave wrong, while right intention required that the purpose of the war be peace and justice, not revenge or conquest. 

Over the centuries, Catholic teaching added further requirements, such as probability of success and proportionality, ensuring that harm caused does not exceed the evil being corrected (CCC, 1997, §§2309–2310).

This framework guided Catholic reflection for centuries, from medieval Europe to the modern world.

From Paul VI to Pope Francis: A Shift Toward Absolute Peace

The 20th century, especially the shadow of nuclear war, changed how the Church thinks about military action. St. Paul VI, speaking at the United Nations in 1965, famously declared: “No more war, war never again!” His message reflected the horrors of World War II, the emerging Cold War, and the threat of nuclear annihilation. 

Paul VI emphasized that war itself was incompatible with the Church’s mission of peace, calling humanity to rediscover nonviolence and peacemaking as central to Christian life.

Pope Francis, in Fratelli Tutti (2020), takes this shift even further. He describes war as “always a defeat for humanity” and critiques the way Just War theory has historically been misused to justify modern violence. Francis observes that new forms of warfare—nuclear weapons, drones, terrorism—make it exceedingly difficult, if not impossible, to meet the criteria outlined by Augustine or Aquinas. 

While he respects the historical framework of Just War, he emphasizes that the Church must now promote just peace and fraternity rather than trying to morally justify war. This echoes Paul VI’s call for “Never Again War” while responding to 21st-century challenges.

Pope Leo XIV, in a recent Angelus address, reinforced this perspective: “So many victims, so many lives and families shattered… Every war is truly a wound inflicted upon the entire human family… Peace cannot be postponed. It is an urgent necessity that must find a home in our hearts and be translated into responsible decisions.” 

Here, our American pontiff stresses that war is not just a political or strategic problem—it is a moral problem affecting the entire human family, and solutions require responsible, ethical leadership, not mere military might.

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