The moral theory of the "just-war" or "limited-war" doctrine begins with the presumption which binds all Christians: We should do no harm to our neighbors. Just-war teaching has evolved as an effort to prevent war. Only if war cannot be rationally avoided does the teaching then seek to restrict and reduce its horrors. It does this by establishing a set of rigorous conditions which must be met if the decision to go to war is to be morally permissible. Such a decision, especially today, requires extraordinarily strong reasons for overriding the presumption in favor of peace and against war. The conditions for a just war are as follows:
Because of the destructive capability of modern technological warfare, the principle of proportionality (and that of discrimination) takes on special significance. Today it becomes increasingly difficult to make a decision to use any kind of armed force, however limited initially in intention and in the destructive power of the weapons employed, without facing at least the possibility of escalation to broader, or even total, war and to the use of weapons of horrendous destructive potential.
"Indeed, if the kind of weapons now stocked in the arsenals of the great powers were to be employed to the fullest, the result would be the almost complete reciprocal slaughter of one side by the other, not to speak of the widespread devastation that would follow in the world and the deadly after-effects resulting from the use of such weapons" (Pastoral Constitution, #80). To destroy civilization as we know it by waging such a "total war" as today it could be waged would be a monstrously disproportionate response to aggression on the part of any nation.
Just response to aggression must also be discriminate; it must be directed against unjust aggressors, not against innocent people caught up in a war not of their making. The Council therefore issued its memorable declaration: "Any act of war aimed indiscriminately at the destruction of entire cities or of extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation."
Side by side with the just-war theory throughout Christian history has been the tradition of nonviolence. One of the great nonviolent figures was St. Francis of Assisi.
While the just-war teaching has clearly been in possession for the past 1,500 years of Catholic thought, the "new moment" in which we find ourselves sees the just-war teaching and nonviolence as distinct but interdependent methods of evaluating warfare. They diverge on some specific conclusions, but they share a common presumption against the use of force as a means of settling disputes. Both find their roots in the Christian theological tradition; each contributes to the full moral vision we need in pursuit of a human peace. We believe the two perspectives support and complement one another, each preserving the other from distortion.
from Catholic Update's condensation of The Challenge of Peace: God's Promise and Our Response, the U.S. Bishops' 1983 historic pastoral on war and peace.