The Catholic Church defines just war in modernity through a rigorous set of conditions that must be met to justify the use of military force 1. These conditions are rooted in the just war theory, which seeks to limit the outbreak of war and govern how it may be waged 2.

Here's a breakdown of the Church's perspective:

1. Conditions for Legitimate Defense:

   • Grave and Lasting Damage: The harm inflicted by the aggressor must be lasting, grave, and certain 1 3.
   • Exhaustion of Alternatives: All other means of resolving the conflict must be shown to be impractical or ineffective 1 3. War should be a last resort after political, economic, and diplomatic efforts have been exhausted 4.
   • Prospect of Success: There must be serious prospects of success in military terms 1 3.
   • Proportionality: The use of arms must not produce evils and disorders graver than the evil to be eliminated 1 3. This condition is heavily influenced by the power of modern means of destruction 1. The recourse to force must be proportionate to the desired result, considering the consequences for people and the planet 5.

2. Key Considerations:

   • Legitimate Authority: The decision to go to war should be made by a legitimate ruler and, where it exists, be authorized by international law or mandate 4.
   • Right Intention: Belligerents should have a rightful intention, aiming for the advancement of good or the avoidance of evil, not for aggrandizement, cruelty, or lust for power 6.
   • Discrimination: There must be an intention to use only legitimate means, distinguishing between combatants and non-combatants, avoiding targeting the population as a whole, and not using weapons of mass destruction 4.

3. Modern Challenges and Perspectives:

   • Increased Destructive Power: The development of nuclear, chemical, and biological weapons, along with new technologies, has given war an uncontrollable destructive power over innocent civilians 7.
   • Risk vs. Benefit: Due to the risks involved, it is increasingly difficult to consider war as a solution, as its risks likely outweigh its supposed benefits 7.
   • Moral Legitimacy: The Catechism of the Catholic Church emphasizes the need for "rigorous conditions of moral legitimacy" to be met for legitimate defense by military force 7.
   • Prudential Judgement: Evaluating these conditions for moral legitimacy requires the careful judgment of those responsible for the common good 1 3.

4. Emphasis on Peace:

   • Promoting Peace: Catholics must work to avoid war and promote peace, recognizing that war indicates a failure to uphold human dignity 8.
   • Preventing Conflicts: Nations should protect human dignity and the right to life by finding more effective ways to prevent conflicts and resolve them peacefully 8.
   • International Law: International law is crucial for building a more human and peaceful world, protecting the weak against the strong 5.

In summary, the Church's definition of just war in modernity maintains the traditional elements of just war theory while emphasizing the need for rigorous scrutiny and the importance of pursuing peaceful solutions whenever possible 1 8. The devastating potential of modern warfare necessitates a cautious approach, with a strong presumption against the use of force 7 9.