Most of our remarks on war must be reserved for a separate section dealing more directly with ‘The Confrontation Between Man and Evil”, which delves into these issues on a deeper level. Here we only intend to provide a limited framework in order to show the comprehensiveness of the Catholic adaptation.
The Church teaches that “insofar as men are sinful, the threat of war hangs over them, and hang over them it will until the return of Christ.”
Yet because of this truth we must never forget that there are “strict conditions for legitimate defense by military force.” These conditions, known as just war doctrine, provide rulers and ruled with a coherent means for evaluating the validity of military action in any conflict. Such a means is vital for any government, particularly in democracies, because in the absence of objective criteria for determining military action, passion and opportunity tend to rule supreme. Ignorance of these principles leads inevitably to atrocities, and the United States, having little regard for Catholic philosophy, is no exception. It violated just war doctrine to great extremes during the Civil War, World War I, and World War II, for example the firebombing of Dresden and the atomic bombings of Hiroshima and Nagasaki. On other points, however, we will see that the United States has behaved with exceptional nobility, and in perfect adherents with certain elements of the doctrine—for example in the treatment of prisoners and many non-combatants. Unfortunately, however, since the doctrine is not acknowledged openly, it is only followed haphazardly and unconsciously, and we have no way of knowing whether this will change tomorrow.
The principles are summarized in the Catechism, paragraph 2302-2317, although their formation goes back to St. Augustine (354-430AD) and continued to be affirmed and developed through St. Thomas Aquinas (1225-1274AD) and later theologians as occasion has arisen. This subject is of particular importance for our contemporaries because of the challenge of terrorism, which has fueled new debates and new questions regarding the nature of legitimate defense. So often we run up against the assumption that our problems, because they are new, cannot be addressed by traditional principles, but that is the beauty of principles: that they are universal, applicable in any age, and need only to be re-applied according to changing circumstances. We will find that this is precisely the case with just war doctrine and the war on terror.
 GS, 78.
 CCC, 2309.