Catholic just war doctrine

Peace on earth is one of the greatest gifts God can bestow on mankind. Peace is not merely the absence of war, but rather, as St. Augustine defines it, the “tranquility of order” (City of God, Book XIX), that is, the harmony within man, among men, and between God and man, that is the fruit of justice and charity. While peace is ultimately a divine gift, every person is called by God to be a peacemaker: “Blessed are the peacemakers, for they shall be called sons of God” (Matthew 5:9). The Church therefore urges everyone to work and pray for peace in the world, while striving to avoid war, violence, and conflict (CCC 2307-2308). 
 
Because men are sinners, however, the threat of war and conflict is always present. Despite the grave dangers of modern warfare, the Church recognizes that nations may have to take up arms as a last resort to defend themselves against an unjust aggressor. While most commentators today justify American involvement in war for strategic reasons, the Church’s just war doctrine offers a moral framework for Christians to assess war in a way that prioritizes peace and the dignity of all persons, while at the same time recognizing a nation’s duty and right to defend its people and protect its homeland. Given our country’s ongoing war with Iran and involvement in other conflicts around the world, it is an opportune time to review Catholic just war doctrine.
 
The just war doctrine is divided into two distinct assessments: The first assessment, ius ad bellum (“right to go to war”), determines whether a nation can be morally justified in going to war. The second assessment, ius in bello (“right conduct in war”), regulates and limits the kind of fighting that is morally permissible during war. As the Catechism of the Catholic Church teaches, this assessment belongs to the prudential judgment of the governmental authority responsible for the common good (CCC 2309), even though these judgments can be challenged and criticized. 
 
There are six criteria that must be met to justify a nation going to war (ius ad bellum): 
 
Legitimate Authority: The first criterion to justify entering war is that the decision must be made by the legitimate governing authority, not by an individual or group that has unlawfully seized or retained control of the state. This is because it is the government’s duty to promote the common good of its citizens. As St. Thomas Aquinas explains, just as it is lawful for the governing authority to have recourse to the sword in defending the common good against internal disturbances by criminals (Rom 13:4), “it is their business to have recourse to the sword of war in defending the common good against external enemies” (Summa theologiae II-II, q. 40, a. 1). 
 
Just Cause: A second criterion for entering war is it can only be for a just cause. The clearest case of a just cause is when a nation acts in self-defense against an unjust aggressor that has attacked it first. Another reason that might justify going to war is to put a stop to severe human rights violations in another country. While there is no clear consensus, some moral theologians and ethicists also contend that a preemptive war might be morally justified if it is necessary to prevent an imminent and certain attack on the homeland. 
 
Right Intention: A third criterion for entering war justly is that those responsible for the decision must have a morally good intention. Right intention consists of correcting the injustice and restoring the peace. War should not be entered into for unjust aims such as territorial expansion, pursuit of strategic advantage, demonstrating military power, or exacting vengeance for past wrongs. 
 
Last Resort: A fourth criterion that must be satisfied is that war must be the last resort. As the Catechism puts it, governments have the right to lawful self-defense “once all peace efforts have failed” (CCC 2308). Engaging in diplomatic negotiations, exacting economic sanctions, issuing ultimatums prior to using force, and other peaceful alternatives must first have been tried and failed before entering war with another country 
 
Serious Prospects of Success: A fifth criterion justifying entry into war is there must be serious prospects of succeeding (CCC 2309). The political leaders responsible for military action must assess whether the aims of the war are achievable. Even for a just cause with all the right intentions, government leaders may nevertheless determine that a war is not winnable. Assessing the probability of success prevents an irrational resort to force that risks wreaking havoc and bloodshed on both sides for no good reason. 
 
Proportionality: A sixth criterion that must be met is proportionality: The expected damage to be inflicted and costs to be born must be proportionate to the good to be achieved. If entering war is expected to cause tremendous harm for relatively little gain, it cannot be justified. The Catechism warns that this consideration is especially important due to the destructive capacities of modern warfare: “The power of modern means of destruction weighs very heavily in evaluating this condition” (CCC 2309).
 
As the Catechism makes clear, the moral law remains intact even during war: “The mere fact that war has regrettably broken out does not mean that everything becomes licit between the parties” (CCC 2312, quoting Vatican II, Gaudium et Spes 79). Once war has begun, there are two chief criteria that must be met to justify how the war is being conducted (ius in bello): 
 
Noncombatant Immunity: The first criterion that must be met for conduct during war is noncombatant immunity. In the Catholic tradition, a state of war permits the use of force only against those who are actively engaged in fighting. The moral law prohibits attacking civilians, prisoners of war, enemy soldiers who have surrendered, medical personnel, the wounded, and all others who are unarmed and do not pose an immediate threat. Due to the increased lethality of modern weaponry and threat of mass devastation, the Church is especially insistent in condemning indiscriminate destruction of civilian populations and non-military targets (CCC 2313-2314). For this reason, Pope Leo rightly condemned as “truly unacceptable” President Trump’s threat to destroy Iran’s “whole civilization” 
 
Proportionality: Proportionality is not just a static principle for determining whether entering war is justified but must constantly be evaluated throughout the war. If the harms and burdens of a war become disproportionately high compared to the good to be achieved, the war can no longer be morally justified. A classic example where the harms became disproportionately greater than the good to be achieved is the Vietnam War, where over time American involvement no longer made sense because the immense destruction, high casualties, and political divisions at home far outweighed its strategic objectives.
 
Catholic just war doctrine recognizes that war may at times be necessary in a fallen world yet can only be morally justified under strict conditions ordered toward peace. Far from justifying violence, it places a heavy burden of proof on those who would wage war and insists that, even in conflict, the dignity of every human person must be upheld. Especially in an age marked by unprecedented destructive power, just war doctrine serves as a guide for conscientious Christians to judge whether war is justified and how it must be fought. Ultimately, it reminds us that war is never an end in itself, but a tragic and imperfect means of last resort to achieve a just and lasting peace. At this time of so much war and violence around the world, let us heed Pope Leo’s constant plea to work and pray tirelessly for peace.