What Is “Just War Theory”?

Morality doesn't disappear because one is engaged in war

Catholic teaching grounds its theory of just war within the context of earthly peace as a great good to be prized and worked for. Peace, the Catechism reminds us, “is not merely the absence of war, and it is not limited to maintaining a balance of powers between adversaries. Peace cannot be attained on earth without safeguarding the goods of persons, free communication among men, respect for the dignity of persons and peoples, and the assiduous practice of fraternity” (CCC 2304). Given that war always involves grave consequences, the reasons for its justification must be seriously discerned. This justification is limited by the following conditions given by the Catholic Church for the legitimate exercise of force:

the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
all other means of putting an end to it must have been shown to be impractical or ineffective;
there must be serious prospects of success
the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition" [CCC 2309].

Determining if these conditions are present belongs to “the prudential judgment of those who have responsibility for the common good.” The Church, while respecting those who serve the defense of their nation for the maintenance of peace, warns against an “anything goes” attitude in times of war. This attitude has become especially dangerous in light of the vast destructive capabilities of modern warfare.

Actions which are crimes because they are “deliberately contrary to the law of nations and to its universal principles” include:

cruel treatment of non-combatants, wounded soldiers, and prisoners;
genocide of a people, nation or ethnic minorities;
indiscriminate destruction of whole cities or vast areas with their inhabitants.

The Church recognizes that “insofar as men are sinners, the threat of war hangs over them and will so continue until Christ comes again,” but also that insofar as charity dissolves the hatred in human hearts, violence will give way to peace.