2309 The strict conditions for legitimate defense by military force require rigorous consideration. The gravity of such a decision makes it subject to rigorous conditions of moral legitimacy. At one and the same time:
2310 Public authorities, in this case, have the right and duty to impose on citizens the obligations necessary for national defense.
2311 Public authorities should make equitable provision for those who for reasons of conscience refuse to bear arms; these are nonetheless obliged to serve the human community in some other way.
2312 The Church and human reason both assert the permanent validity of the moral law during armed conflict. “The mere fact that war has regrettably broken out does not mean that everything becomes licit between the warring parties.” . . .
2318 “In [God’s] hand is the life of every living thing and the breath of all mankind” (Job 12:10).
2319 Every human life, from the moment of conception until death, is sacred because the human person has been willed for its own sake in the image and likeness of the living and holy God.
2320 The murder of a human being is gravely contrary to the dignity of the person and the holiness of the Creator.
2321 The prohibition of murder does not abrogate the right to render an unjust aggressor unable to inflict harm. Legitimate defense is a grave duty for whoever is responsible for the lives of others or the common good.