War is Hell: Just War Theory Explained

A quick look at Just War Theory

War is Hell

From the outset, I want to make it clear: I am not attempting to comment on the ongoing conflicts in the world (Iran and Israel, Iran and the U.S., Israel and Hamas, Russia and Ukraine, etc.). Everyone has an opinion and no one has the full picture. However, I am sure that all of you reading this have ideas about those conflicts. You have predictions about what might happen. And you are likely discussing these matters with your friends, coworkers, and families. What I do not hear a lot about is just the stark reality: war is hell. We think about war as if it is a video game or a game of chess between politicians. Even the nature of warfare seems to be obscured; often we hear of drone strikes and missiles flying, but the idea of boots on the ground is simply not as visceral as reality when read about in the news reports.

The Catholic Church, as always, has something to say. In a moment, I will be outlining the main movements of just war theory, formulated and popularized by St. Thomas Aquinas. Pope Leo and Pope Francis both spoke of the current regional conflicts as a “piece meal Third World War” and condemned it. Pope Francis, speaking of modern contexts in 2022, dramatically said—and he was not making a doctrinal point here but rather a rhetorical one—that “there is no such thing as just war: They do not exist!” Pope Leo, in May 2025, spoke in his First Sunday Address calling for “no more war” and that there be a ceasefire in Ukraine and a “just, authentic, and lasting peace.” Echoing the words of Pope Pius XII in 1939 at the outset of WWII, Pope Leo said that “nothing is lost with peace; everything can be lost with war.” Why did he say this? Are there not examples in the past of wars which needed to be fought? In 1939, was it not evident that the Nazi threat needed to be met with like-force and put down permanently? Why does the voice of the peacemaker reiterate the universal need for peace, stability, and tranquility? Because War is Hell. Man is not made to yearn for Hell but for Heaven.

Before speaking of Just War Theory, I want to make it clear that I am speaking about war as organized, violent geopolitical conflict between nations or factions within a state with the objectives that are often political, economic, territorial, or ideological. The war in heaven, shown us in Revelation 12, between the angels and demons is fundamentally different. It is a spiritual, metaphysical conflict that is rooted in a fundamental rebellion against God Himself. Such a “war” is unlike any human war. I bring this up only to further clarify what the Church means by “war” in Just War Theory. Certainly, the fight between Satan and St. Michael was a just fight. The rebellion of the demons required a forceful and sustained response by the angels loyal to the Lord. Likewise, this spiritual warfare is one in which all the faithful are called to engage. You and I are called to fight the world, the flesh, and the devil as it pertains to sin. Spiritual warfare is requisite for the lover of Christ precisely because the goal is Heaven and the means along the way is opposed to Hell.

Just War Theory

Just War Theory is a framework for evaluating the ethical and moral permissibility of conducting warfare or entering into war. St. Augustine and St. Thomas Aquinas are the primary thinkers in this arena that expound Christian theology on the topic, but later secular thought continues to reflect on it—think, for example, of the Geneva Conventions of war. The theory looks at criteria to determine when a war is justified and how it should be carried out. There are three main categories of Just War Theory: the Justice of War, in War, and after War.

Jus ad Bellum (Justice of War)

First, the war must address a serious wrong, such as aggression or grave injustice. In other words, there must be a just cause. For example, self-defense would be a just cause if a nation was invaded. The war against the Nazi forces following the blitzkrieg is an easy to see example of this.

Individual factions in a nation cannot declare war. Only duly constituted authorities and governments have the legitimate authority to declare war.

The aim of the war must itself be peace and justice. The right intention would preclude ulterior motives such as conquest, revenge, and strategic capture of resources.

War must always be the last resort. All other reasonable nonviolent options must be exhausted.

The anticipated and foreseen benefits of a war must outweigh the expected harms. This is the principle of proportionality in moral theology.

Finally, there must be a reasonable chance of success. In order to have a just cause, there must be a realistic prospect of achieving that goal.

Jus in Bello (Justice for War)

Non-combatants must not be targeted—this is the principle of discrimination in which the forces distinguish between legitimate military targets and civilians, medical personnel, and chaplains. Non-combatants must be spared. Of course, in war, there will be unexpected and unintended collateral damage, but this must be reasonably mitigated.

The principle of proportionality during a war refers to the harm caused by military actions. Simply put, the harm must not outweigh the military advantage to be gained. To take this a step further, an evil action cannot be done to later achieve a good end. We could think here of the use of nuclear weapons or carpet (indiscriminate) bombing of civilian centers—these would violate discrimination and proportionality.

The actions taken must be of necessity to achieve legitimate military objectives. Gratuitous destruction should be avoided.

Jus post Bellum (Justice after War)

This third category is not part of St. Thomas’s Just War Theory, and is what might be considered “emerging” in terms of the modern notion of Just War Theory, but it is worth mentioning. The fair treatment of defeated parties, including reconstruction and reconciliation, must be considered. There must also be just accountability for war crimes and real effort to restore justice.

Just War Theory: Out of Date?

Some argue that Just War Theory is out of date in modern warfare for two reasons. First, it can be manipulate to justify aggression, and, second, it does not explicitly take into account things like drones and cyberattacks. Generally, I hold that Just War Theory has its place as a way to restrain violence. Human beings have been trying to justify sin since the Fall. Therefore, solid unflappable principles gives us a language to share as we condemn injustice.

It is also odd to speak of Just War Theory as outdated when modern international law was shaped by it. The Geneva Conventions of 1949 include the principles of discrimination and proportionality specifically. The International Criminal Court (ICC) is unfortunately manipulated by various powers unjustly, but in principle can investigate conflicts and assess accountability based on Just War principles. The 1945 UN Charter restricts war to self-defense or actions authorized by the Security Council (self-defense and legitimate authority).

There are certainly issues with Just War Theory that cannot be ignored. For example, the principles are far less clear in asymmetric warfare where non-state actors like terrorist groups or insurgencies are active. In modern urban warfare, combatants blend in with civilians making discrimination nearly impossible. In the event of nuclear or total war where the entire society is engaged or targeted is simply not envisioned in Just War Theory which is based on conventional warfare on the field of battle. There are also modern means like economic sanctions, propaganda, and proxy wars which add complexity to an already complex situation.

Moral theologians have the task to update Just War Theory for modern times, and are hard at work doing so. This will prove fruitful if it gives us the necessary restraint and common language to avoid warfare. Just War Theory has been manipulated to justify the Invasion of Iraq in 2003 and the Invasion of Ukraine in 2022. Neither conflict actually meets the standards for going to war. In an era of advanced technology and non-conventional threats, I think Just War Theory becomes more important, not less. It simply needs to be adapted and updated, which happens often with the application of principles to specific circumstances. The principles themselves do not change, but the applications can be vastly different over the centuries.

What should be clear to all is that war is Hell. Peace and justice must be our goal. There is no room for war in a modern, civilized society. How, then, can we convince those who would seek war for their own gain that they are doing the work of Satan? The only answer with any real staying power is: Jesus Christ. You and I must step up our evangelistic efforts. Sharing the Gospel is the only way to lead the world towards the peace that only He can give.

In this time of uncertainty and widespread regional conflicts, we are on the precipice of world war. Pray for those who are already fighting, especially the troops who are fighting for peace and justice. Pray for conversion for those who fight for their own selfish interest detached from what is good and true. Pray for those living in affected areas who have lost homes, family, livelihood, and a means of reliable food and clean water. War is Hell and our brothers and sisters throughout the world need our support. This begins with prayer and doing the work of evangelization with those nearest you.