The Catholic Church acknowledges the legitimacy of just war teaching when defending the innocent in the face of grave evil, but emphasizes the importance of working to avoid war and promote peace 1. The Church recognizes the cost of war and its harm to human life 1.

Here's a breakdown of the key elements of the Church's teachings on just war:

Just Cause

• Defense against aggression: War is only justified to oppose a great evil that cannot be effectively opposed by any other means 2. It should be an act of self-defense, not aggression 2.
• Protecting the common good: Legitimate political authorities may resist unjust aggression to defend the common good and protect innocent life 3.
• Response to grave evil: Nations have the right and obligation to defend human life and the common good against terrorism, aggression, and similar threats, such as persecution based on religion 1.

Legitimate Authority

• Sovereign authority: The authority to declare and wage war rests with those who hold supreme authority, not private individuals 4.
• Proper intention: Belligerents should intend the advancement of good or the avoidance of evil, not for aggrandizement, cruelty, or lust for power 4.
• Rightful intention: Actions should be for the sake of the stated cause, not for strategic or economic gain, empire, or conquest 2.

Conditions for Just War

• Last resort: The decision to go to war should be taken as a last resort, after all other political, economic, and diplomatic means have been exhausted 2.
• Probability of success: There must be a reasonable prospect of success in military terms 2.
• Proportionality: War should be confident of achieving its objective without causing more harm than the evil it opposes 2.
• Legitimate means: There must be an intention to use only legitimate means, such as discriminating between combatants and non-combatants, not targeting the population as a whole, and not using weapons of mass destruction 2.
• Moral law: The moral law remains permanently valid during armed conflict, meaning not everything becomes permissible between warring parties simply because war has broken out 5.

Concerns and Modern Warfare

• Grave evils: All wars bring great evils, and few wars bring great benefits 3.
• Limits to force: Using more force than necessary is unjust 6.
• Unacceptable actions: Deliberately killing those not engaged in unjust aggression, such as unarmed prisoners of war, is unacceptable. Indiscriminate bombing and targeting schools, hospitals, or residential areas are also considered acts of terror, not war 7.
• New technologies: The development of nuclear, chemical, and biological weapons, along with new technologies, has granted war an uncontrollable destructive power over innocent civilians 8.
• Preventive war: The Church has raised fundamental moral concerns about the preventive use of military force 1.
• Difficult application: Due to the destructive nature of modern warfare, practically no conditions exist for a just launching of war, especially with the potential to destroy life on earth 9. It is very difficult nowadays to invoke the rational criteria elaborated in earlier centuries to speak of the possibility of a “just war” 8.

The Church acknowledges the right to self-defense but insists on strict conditions for the use of military force, especially in light of modern warfare's destructive capabilities 98.