“Certainly, war has not been rooted out of human affairs.  As long as the danger of war persists and there is no international authority with the necessary competence and power, governments cannot be denied the right of lawful self-defense, once every means of peaceful settlement has been exhausted.”
Gaudium et Spes #79

While war inevitably is the result of sinful disorder in the relations between peoples and nations, in a fallen world, the Catholic Church maintains, war can, at least in some limited situations, be a justified means of restraining evil and protecting the innocent.   This just war teaching of the Catholic Church, rooted in the principle of legitimate self-defense, has developed as part of a larger effort to prevent war, or if war is unavoidable, to limit its violence and destructiveness. It establishes a set of rigorous conditions that must be met if a state’s decision to go to war (jus ad bello) and its conduct of a war (jus in bello) is to be morally justifiable.  The presumption of just war teaching is overwhelmingly  in favor of peace and against the recourse to war.(1)