Okinawa, Japan, 1951. Photograph: Nelse Einwaechter/Corbis

A word about official Catholic teaching on war and peace: While Christian theorists historically backed crusades as well as the just-war tradition and pacifism as options for responding to conflict, in the modern era Catholic teaching has considered only the latter two options to be legitimate.

Early Christianity tended toward nonviolence, in part because violence appeared to be a violation of charity and in part because participation in the military involved worship of Caesar, which constituted idolatry. In the fourth century, the emperor Constantine made Christianity the official religion of the Roman Empire, and in so doing engendered a Christianity more closely aligned with the state and therefore with war-fighting. Official Christianity dealt with this development socially and ecclesiologically by designating a certain class of persons—those who were members of religious orders—to bear witness to the kingdom of God through their nonviolence, and with a few exceptions this division held in practice. A number of lay movements espoused nonviolence, but official Catholicism generally frowned on them: As late as 1956, Pius XII stated, in his Christmas address, that laypersons could not be conscientious objectors to what state and Church deemed a just war.

The just-war tradition was never formally declared Catholic doctrine but achieved the force of history by the accrual of writing and teaching over time. In its Christian context, the tradition first found articulation in the writings of St. Ambrose and St. Augustine in the fourth and fifth centuries. It was later refined by St. Thomas Aquinas (1225–74) and others.

The just-war tradition arose out of the attempt to answer the questions of whether war could be conducted as a legitimate means for pursuing peace and, if so, under what moral limitations. Certain principles define the limits to be placed on war if fighting is to have peace as its true end. The principles are divided into those that must be met in order to engage in war in the first place (jus ad bellum) and those that must be met in the conduct of war (jus in bello).

Under the ad bellum rubric, the first principle that must be met is “just cause.” The tradition in the modern era limits what can count as a just cause, allowing only wars of self-defense and humanitarian intervention, and ruling out wars of retribution, once considered just. Second, war must be declared by a legitimate authority or competent authority, making a war unlawful when declared by private individuals. Third, there must be right intention in going to war; this principle concerns the objective purpose of the war, such as the preservation of human rights. Fourth, war must be a last resort. All reasonable means of nonviolent conflict resolution must be exhausted before recourse to arms. Fifth, there must be a reasonable chance of success or probability of success if the resort to violence is not to be gratuitous. Here, success is more than military victory; it includes the restoration of a proximate peace that is the aim of a just war. Sixth, there must be proportionality between the overall destruction of a war and the good that the war seeks to achieve. Danger lies in escalating portrayals of the evil against which one is fighting, thereby rationalizing the escalation of violence beyond what is truly proportionate.

There are two in bello criteria that must be met during the conduct of war. The first is, again, proportionality. Here, the focus is on specific tactics. For instance, it is possible to hold that the Persian Gulf War of 1991 was, overall, a just war, while saying that the intensive bombing of retreating Iraqi soldiers on the road to Basra near the war’s end was, among other things, disproportionate. Second, the conduct of war must admit noncombatant immunity or discrimination. Persons not directly involved in the war-fighting effort are not to be victims of its violence.

Vatican II (1962–65) declared that while nations are justified in using
lethal force if necessary to defend their borders, individuals in any given nation can legitimately be pacifist. This declaration was in response to and further enabled the living practice of nonviolence by Catholics.

Todd D. Whitmore teaches social ethics in the theology department at the University of Notre Dame. His essay is drawn from Modern Catholic Social Teaching: Commentaries & Interpretations (2005), edited by BC theologian Kenneth B. Himes, OFM, and reprinted by permission of Georgetown University Press.