The Catholic Church's teaching on war is rooted in the "just war" doctrine, which aims to limit the outbreak of war and govern its conduct 1. This doctrine acknowledges that while war always involves the loss of life and causes great hardship, legitimate political authorities have the right to resist unjust aggression to defend the common good and protect innocent life 1.

Conditions for a Just War

For a war to be considered just, it must meet several rigorous conditions of moral legitimacy 2 3:

• Just Cause: The war must be waged to oppose a grave evil that cannot be effectively countered by any other means. It must be an act of self-defense, not aggression 4. This means that those who are attacked deserve it due to some fault, and the war avenges wrongs, such as a nation refusing to make amends for wrongs inflicted or to restore what it has unjustly seized 5.
• Last Resort: All other political, economic, and diplomatic means must have been exhausted before resorting to war 4 3. Dialogue and negotiation are considered the best channels for resolving disputes between nations 6 7.
• Legitimate Authority: The decision to go to war must be made by a legitimate ruler or sovereign state, and where applicable, authorized by international law or mandate 4 5 8.
• Prospect of Success: There must be a reasonable prospect of achieving the objective without causing more harm than the evil it opposes 4 3.
• Proportionality: The use of arms must not produce evils and disorders graver than the evil to be eliminated 3. The destructive power of modern weapons weighs heavily in evaluating this condition 9 3. Pope Francis notes that it is very difficult to invoke the rational criteria for a "just war" today, given the uncontrollable destructive power of modern weaponry 2.
• Right Intention: The intentions of those fighting the war must be upright, acting for the stated cause and not for strategic or economic gain, empire, or conquest 4 5. There must also be an intention to use only legitimate means, such as discriminating between combatants and non-combatants, and not targeting the population as a whole or using weapons of mass destruction 4.

Protection of the Vulnerable

The protection of the vulnerable is a central tenet of Catholic social teaching and is intrinsically linked to the justification for war. The Church emphasizes that war should always be a last resort to defend the innocent in the face of grave evil 10.

Key aspects of protecting the vulnerable in the context of war include:

• Defense Against Aggression: States have the right and duty to organize a defense, even using armed force, in the tragic case of a war of aggression 9. This is to protect innocent life and the common good 1.
• Non-Combatant Immunity: The moral law remains valid during armed conflict, and "the mere fact that war has regrettably broken out does not mean that everything becomes licit between the warring parties" 11. This includes discriminating between combatants and non-combatants and not targeting the civilian population 4. Actions designed for the methodical extermination of an entire people, nation, or ethnic minority are vehemently condemned as horrendous crimes 12.
• Prohibition of Certain Weapons and Tactics: The use of weapons of mass destruction is prohibited 4. The Church also raises fundamental moral concerns about the preventive use of military force and rejects torture as fundamentally incompatible with human dignity 10 13.
• Humanitarian Concerns: International agreements aimed at making military activity and its consequences less inhuman, such as those concerning the treatment of wounded soldiers and prisoners, must be honored and improved upon 12 6.
• Conscientious Objection: The Church recognizes the moral right to conscientious objection to war in general, a particular war, or a military procedure, provided that those who object agree to serve the human community in some other way 12 10.
• Post-Conflict Reconciliation: Nations should promote reconstruction and reconciliation in the wake of conflicts 10.

Modern Challenges to Just War Theory

While the traditional elements of just war doctrine remain, modern warfare presents significant challenges to their application. The destructive power of contemporary weapons makes it increasingly difficult to ensure that the use of arms will not produce evils graver than the evil to be eliminated 2 14 3. Pope Francis has stated that "never again war!" because its risks will probably always be greater than its supposed benefits 2. The Church encourages the work of peace and the building of competent international authorities to prevent warfare 1.

In summary, a just war is a highly restricted concept in Catholic teaching, permissible only under stringent conditions as a last resort for legitimate self-defense and the protection of the vulnerable, with a clear emphasis on proportionality and right intention. The Church consistently advocates for peaceful means of conflict resolution and highlights the immense human cost of war.