Self Defense in the Catechism of the Catholic Church

"The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. 'The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor. . . . The one is intended, the other is not.' " (CCC 2263)

"Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow: "If a man in self-defense uses more than necessary violence, it will be unlawful: whereas if he repels force with moderation, his defense will be lawful. . . . Nor is it necessary for salvation that a man omit the act of moderate self-defense to avoid killing the other man, since one is bound to take more care of one's own life than of another's." (CCC 2264)

"All citizens and all governments are obliged to work for the avoidance of war. However, "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 all peace efforts have failed." (CCC 2308)