The Catholic Church and Its Stance on War

Catholicism’s stance on war is built on the Just War Theory, which says that all things being equal, the state has a right to wage war — just like it has a right to use capital punishment. However, just like with capital punishment, the right to wage war isn’t an absolute right.

The basis of the Just War Theory is the natural moral law, and it incorporates a moral evaluation before going to war (the reasons for it) and during it (the means used). Everything leading up to war and every act during it must fulfill certain criteria, otherwise, the conflict is judged to be an immoral war.

The Church believes that throughout history some wars were morally right, but many wars could’ve and should’ve been avoided.

The Just War Theory can be broken down into two components:

Ius ad bellum (Latin for right to war or moral reasons that justify a country's going to war)
Ius in bello (Latin for right in war or moral conduct during war).

The two components above are further elaborated in this way:

Before war:

Just cause (ius ad bellum): The reasons for going to war must be morally correct, for example to repel invading enemy forces.
Competent authority (ius ad bellum): Only legitimate, authentic, and authorized leaders can declare and involve the nation in war. Private citizens, corporations, special interest groups, associations, political parties, and so on, have no moral authority to declare war.
*Comparative justice (ius ad bellum): The values at stake must be worth the loss of life, the wounding of others, the risk of innocent victims and damage to property.
*Right intention (ius ad bellum): Acceptable reasons for going to war are a just cause, such as the stopping of an unjust aggressor, or having the goal of restoring peace rather than seeking revenge, retaliation, or total destruction of the enemy (without any possibility of surrender).
*Last resort (ius ad bellum): All viable alternatives must be exhausted before resorting to war. Going to war shouldn’t be the first step but the last one.
*Probability of success (ius ad bellum): A just war demands that the hope of winning the war is reasonable. Fighting just to make or prove a point or merely defending honor is foolish.
Proportionality (ius ad bellum): The evils and suffering that result from the war must be proportionately less or smaller than the evils or suffering, which would have ensued had there been no conflict.

During war:

Proportionality (ius in bello): A just war uses moral means during the execution of the war. Biological weapons are considered immoral, because they disproportionately harm more people and in more severity than is necessary for victory. Tactical nuclear weapons are only permissible if employed as a last resort, and there is significant accuracy and control to target only valid sites.
Discrimination of noncombatants (ius in bello): Military and strategic targets are the only morally permissible sites for attack. Civilian population centers and any place where noncombatants reside shouldn’t be targeted.