Just War Theology

[Jesus said:] “You have heard that it was said, ‘Eye for eye, and tooth for tooth.’ But I tell you, Do not resist an evil person. If someone strikes you on the right cheek, turn to him the other also. And if someone wants to sue you and take your tunic, let him have your cloak as well. If someone forces you to go one mile, go with him two miles. Give to the one who asks you, and do not turn away from the one who wants to borrow from you.
“You have heard that it was said, ‘Love your neighbor and hate your enemy.’ But I tell you: Love your enemies and pray for those who persecute you, that you may be sons of your Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your brothers, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your heavenly Father is perfect.”
—Matthew 5:38-48, NIV

In the beginning, all Christians were pacifists. There were two reasons for this. First, they interpreted the teachings of Jesus as prohibiting violent acts, because of the Scripture I just quoted. Second, joining the Roman Army entailed taking an oath to the pagan gods of Rome, which included the Emperor. That was an act of blasphemy and idolatry that the Church could not tolerate and that no Christian would do.

This problem did not affect Jews, because neither situation arose for them. They did not feel bound by the teachings of Jesus, and they never had to deal with the problem of service in the Roman Army. Unlike Christianity, Judaism was a legal religion, and the Romans had given the Jews an exemption from military service.

Pacifism served the Church quite well when it consisted mainly of working-class people and slaves, but by the early part of the fourth century, Christianity had not only become legal, it had spread to all social classes, including political leaders. Christian theologians of that era were not engaged in theoretical scenarios. If for no other reason, it was an immediate and pressing problem because of the way the Roman court system worked. Today, if they summon you to testify in court, they make you take an oath to make sure that you are telling the truth. In Roman courts—even at the time of Augustine—they routinely tortured the witnesses to ensure they were telling the truth! In those circumstances, what does the Church do if a judge converts to Christianity? Or for that matter, how does a Christian policeman apprehend a violent criminal? How does a Christian ruler deal with an armed insurrection or a military invasion?

Christian theologians were therefore dealing with a pressing, practical issue: if Christians are in political leadership roles, how do they run the police force, the army, and the court system? Because they were working against a background of pacifism, they tried to minimize the use of violence as much as possible.


Augustine was among the first Christian theologians to wrestle with the use of violence to maintain public order. He took his cue from the Old Testament rule of an eye for an eye, which he interpreted as limiting violence: It is an eye for an eye, not a life for an eye. So Augustine reasoned that the only justifiable purpose for waging a war is to bring peace, and that one must use the least amount of force that is necessary.


According to Thomas Aquinas, the purpose of civil government is to not to prevent evil, but to bring about justice. Of course one must eliminate evil to bring about justice, but eliminating evil is in the middle of the process, not at the end. The end goal is justice. For Aquinas, war is justified only if the good it brings outweighs the harm it causes—on both sides. Because Jesus commands us to love our enemies, if we wage war, the result must leave our enemy in a better state than if we had not waged war. Aquinas felt that, on a personal level, self-defense is not a justification for violence. That means you cannot fight back if a bully attacks you, but you can fight a bully to rescue another person. If you use violence, whether you are going to war or defending a person from bullies, you cannot have the intention of killing the evildoers, you can only have the intention of stopping them.


Luther was an Augustinian monk, so it is no surprise that he agrees with Augustine. In Luther’s day, rulers were Christians and maintaining social order was a problem. Luther believed that civil government has the authority to impose violence in order to restore social order.


Calvin was younger than Luther and their lives overlapped. He was the earliest champion of the power of the civil government, because he was part of the civil government of Geneva, charged with reforming the church and cleaning up social chaos. He believed that civil government had the right to require such things as church attendance and to punish such things as heresy. Calvin departed from ancient Christianity by teaching that you can only criticize the government’s use of violence for these purposes if the government is committing blasphemy. Calvin believed that pacifism is blasphemy, while the ancient church believed it was the only Christian thing to do.

Anabaptists—the ‘Free Church’ Movement

The Anabaptists arose in Switzerland in the 16th century. They are most known for baptizing adult converts, and since everyone in Europe had been baptized as a child at that time, they were called Anabaptists, which is Greek for “re-baptizers.” They got into a lot of trouble because baptism had a legal as well as a religious meaning. Many of them had discarded all Christian holy days and customs under the mistaken impression that the Roman Catholic Church had invented them—but what is important to us in this context is that they took part in setting up a religious dictatorship in the German city of Münster. It was a horrible disaster, and in reaction and in repentance, they became pacifists. That is why many sects that originated from them are ‘peace churches’ today—such as the Amish and the Mennonites.


I’ve given you a rather superficial tour of historic western Christian theologians on this topic.

Roman Catholics are the best systematic thinkers in Christendom, so they have most coherent teaching on this topic, which falls under the heading of Christian moral theology. They have formulated a Just War theory that has become the international standard for starting and waging wars. Protestant seminaries teach it in ethics classes, and you will even find it codified in international law, such as the Geneva Convention.

Just War theory has two parts, each with a Latin name:

  • Jus ad bellum—justice in going to war.
  • Jus in bello—justice in conducting a war that is in progress.

For a war to be considered a just war, it has to meet all of the criteria. These criteria were thought up long ago, so this is not a comment on any specific war that may be going on as you are reading this. Most wars do not meet these criteria, because the criteria were designed to prevent as much war as possible—and that goes back to the pacifist origins of Christianity. My purpose is not to depict any given war as just or unjust, but to inform you about the criteria for a just war so that you can formulate and defend your opinion in the light of historic Christian moral theology.

I put this together rather quickly because it was topical at the time of writing.

Jus ad bellum—entering into a war

Let us assume that you are the absolute monarch of Outer Petunia, and you are worried about the military intentions of the Duchy of Ambrosia. You are worried that you might be involved in a war, so you want to think out that scenario in advance, so that if it arises, you’ll be doing the right thing.

To enter into a just war, you have to satisfy all of the following seven criteria:

  • You must have a just cause.
  • Whatever reason you may have for going to war, it is only a just cause if the war is absolutely necessary for life to continue. Inconvenience, danger, and threats are not enough to justify a war.
  • You must have the right intention.
  • The only right intention for a war is to restore peace that has been disturbed or to impose order on disorder. Inflicting punishment or causing harm are not right intentions. Revenge and vandalism are not ‘right intentions.’
  • You must have legitimate authority.
  • You must be the legal authority in your nation that has the ability to declare war. For example, in the United States, the Constitution gives Congress the legal authority to declare a war. In this example, you are the absolute monarch of Outer Petunia, so you have the authority to declare and wage war.
  • The war must be proportional to the problem.
  • This means, for instance, that if the ruler of another nation simply insults you, you cannot go to war. It also means that if your adversary sends troops to take over one of your provinces, you can go to war to get it back, but you have to stop there. Anything more is not proportional to the problem. The good that results from the war must outweigh the evil—on both sides. So that means that the war must ultimately benefit your adversary as well as yourself.
  • The war must be your last resort.
  • All other means to settle the problem must have failed before you go to war. Even if your adversary’s army is marching toward your border, you must seek a diplomatic solution first, and you must exhaust all other possibilities before you resort to war.
  • You must have a reasonable hope of success.
  • If it is obvious that no matter what you do, you will lose the war, then you are not justified in going to war, because it would cause more harm than good. Success is more than just a military victory. Success means accomplishing the goal of the war, and the only acceptable goal is to restore peace, to restore order, and thus to benefit not only yourself, but your adversary. The result must be, in short, a better world. If you win a military victory, but create a greater mess than the one you cleaned up, your military victory does not constitute a success.
  • Your cause must be more in the right than your adversary’s cause.
  • If you are attentive, you’ll notice that this principle concedes that even your adversary is right on a point or two. However, it is not a just war for you if you are not right on more points than your adversary is.

Traditionally, it is only a just war if it is in response to violence, because of the proportionality principle. War is only proportional to the problem if the problem is a war. Therefore, under most circumstances, whoever fires the first shot cannot be fighting a just war.

Notice that in an actual war, people can differ about whether or not these criteria have been met. The purpose of these criteria is not to judge wars, but to guide decision makers when they are thinking about war. The goal is to reduce the number of wars.

Jus in bello—justice in the Conduct of War

Let’s go back to your role as absolute monarch of Outer Petunia. Things have changed since you started reading this. You are now under attack by the Duchy of Ambrosia. You and your advisers have carefully thought through all the criteria above. All your diplomatic efforts to resolve the crisis have failed. The Duke of Ambrosia has sent your ambassadors home and cut off all channels of communication. You have decided that war, your last resort, is now unavoidable.

Now how do you go about conducting the war? There are two principles for justice in the conduct of war, both of which you must meet:

  • Your actions must be proportional to your combat objectives, and they must produce more good than evil for both sides.
  • For example, if you need to bomb a bridge to prevent enemy troop movements, you must use the smallest and most accurate bomb for that purpose, and you must minimize casualties on both sides.
  • You must discriminate between soldiers and noncombatants.
  • For example: If you want to bomb a bridge to prevent troop movements, you bomb it at night, not during rush hour. You cannot randomly bomb a civilian population to terrorize them no matter what the purpose. You cannot shoot medics.

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