The Just War Theory of the Catholic Church

A crusader is shot by a Muslim warrior during the Crusades
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The Catholic Church’s teaching on just war developed very early. St. Augustine of Hippo (354-430) was the first Christian writer to describe the four conditions that must be met in order for a war to be just, but the roots of just-war theory go back even to non-Christian Romans, particularly the Roman orator Cicero.

Two Types of Justice Concerning War

The Catholic Church distinguishes between two types of justice concerning war: jus ad bellum and jus in bello. Most of the time, when people discuss just-war theory, they mean jus ad bellum (justice before the war). Jus ad bellum refers to those four conditions described by Saint Augustine through which we determine whether a war is just before we go to war. Jus in bello (justice during the war) refers to how the war is conducted once a just war has been started. It is possible for a country to fight a war that meets the jus ad bellum conditions for being just, and yet to fight that war unjustly—by, for example, targeting innocent people in the enemy’s country or by dropping bombs indiscriminately, resulting in the deaths of civilians (commonly known by the euphemism collateral damage).

Just War Rules: The Four Conditions for Jus Ad Bellum

The current Catechism of the Catholic Church (para. 2309) defines the four conditions that must be met in order for a war to be just as:

  1. the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
  2. all other means of putting an end to it must have been shown to be impractical or ineffective;
  3. there must be serious prospects of success;
  4. the use of arms must not produce evils and disorders graver than the evil to be eliminated.

These are hard conditions to fulfill, and with good reason: The Church teaches that war should always be the last resort.

A Matter of Prudence

The determination of whether a particular conflict meets the four conditions for a just war is left to the civil authorities. In the words of the Catechism of the Catholic Church, “The evaluation of these conditions for moral legitimacy belongs to the prudential judgment of those who have responsibility for the common good.” In the United States, for instance, that means Congress, which has the power under the Constitution (Article I, Section 8) to declare war, and the President, who can ask Congress for a declaration of war.

But just because the President asks Congress to declare war, or Congress declares war with or without the President's request, does not necessarily mean that the war in question is just. When the Catechism states that the decision to go to war is ultimately a prudential judgment, that means that the civil authorities bear the responsibility for making sure that a war is just before they fight it. A prudential judgment does not mean that a war is just simply because they decide that it is so. It is possible for those in authority to be mistaken in their prudential judgments; in other words, they may consider a particular war just when, in fact, it may be unjust.

More Just War Rules: The Conditions for Jus in Bello

The Catechism of the Catholic Church discusses in general terms (para. 2312-2314) the conditions that must be met or avoided while fighting a war in order for the conduct of the war to be just:

The Church and human reason both assert the permanent validity of the moral law during armed conflict. "The mere fact that war has regrettably broken out does not mean that everything becomes licit between the warring parties."
Non-combatants, wounded soldiers, and prisoners must be respected and treated humanely.
Actions deliberately contrary to the law of nations and to its universal principles are crimes, as are the orders that command such actions. Blind obedience does not suffice to excuse those who carry them out. Thus the extermination of a people, nation, or ethnic minority must be condemned as a mortal sin. One is morally bound to resist orders that command genocide.
"Every act of war directed to the indiscriminate destruction of whole cities or vast areas with their inhabitants is a crime against God and man, which merits firm and unequivocal condemnation." A danger of modern warfare is that it provides the opportunity to those who possess modern scientific weapons—especially atomic, biological, or chemical weapons—to commit such crimes.

The Role of Modern Weaponry

While the Catechism mentions in the conditions for jus ad bellum that “the use of arms must not produce evils and disorders graver than the evil to be eliminated,” it also states that “The power of modern means of destruction weighs very heavily in evaluating this condition.” And in the conditions for jus in bello, it is clear that the Church is concerned about the possible use of nuclear, biological, and chemical weapons, the effects of which, by their very nature, cannot easily be confined to combatants in a war.

The injury or killing of the innocent during war is always forbidden; however, if a bullet goes astray, or an innocent person is killed by a bomb dropped on a military installation, the Church recognizes that these deaths are not intended. With modern weaponry, however, the calculation changes, because governments know that the use of nuclear bombs, for instance, will always kill or injure some who are innocent.

Is Just War Still Possible Today?

Because of that, the Church warns that the possibility of the use of such weapons must be considered when deciding whether a war is just. In fact, Pope John Paul II suggested that the threshold for a just war has been raised very high by the very existence of these weapons of mass destruction, and he is the source of the teaching in the Catechism.

Joseph Cardinal Ratzinger, later Pope Benedict XVI, went even further, telling the Italian Catholic magazine 30 Days in April 2003 that "we must begin asking ourselves whether as things stand, with new weapons that cause destruction that goes well beyond the groups involved in the fight, it is still licit to allow that a 'just war' might exist."

Furthermore, once a war has begun, the use of such weapons may violate jus in bello, meaning that the war is not being fought justly. The temptation for a country that is fighting a just war to use such weapons (and, thus, to act unjustly) is one reason why the Church teaches that “The power of modern means of destruction weighs very heavily in evaluating” the justice of a war.

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ThoughtCo. "The Just War Theory of the Catholic Church." Learn Religions, Sep. 15, 2021, ThoughtCo. (2021, September 15). The Just War Theory of the Catholic Church. Retrieved from ThoughtCo. "The Just War Theory of the Catholic Church." Learn Religions. (accessed May 30, 2023).