Gaudium et Spes §79

⁷⁹ Even though recent wars have wrought physical and moral havoc on our world, the devastation of battle still goes on day by day in some part of the world. Indeed, now that every kind of weapon produced by modern science is used in war, the fierce character of warfare threatens to lead the combatants to a savagery far surpassing that of the past. Furthermore, the complexity of the modern world and the intricacy of international relations allow guerrilla warfare to be drawn out by new methods of deceit and subversion. In many causes the use of terrorism is regarded as a new way to wage war. Contemplating this melancholy state of humanity, the council wishes, above all things else, to recall the permanent binding force of universal natural law and its all-embracing principles. Man's conscience itself gives ever more emphatic voice to these principles. Therefore, actions which deliberately conflict with these same principles, as well as orders commanding such actions are criminal, and blind obedience cannot excuse those who yield to them. The most infamous among these are actions designed for the methodical extermination of an entire people, nation or ethnic minority. Such actions must be vehemently condemned as horrendous crimes. The courage of those who fearlessly and openly resist those who issue such commands merits supreme commendation. On the subject of war, quite a large number of nations have subscribed to international agreements aimed at making military activity and its consequences less inhuman. Their stipulations deal with such matters as the treatment of wounded soldiers and prisoners. Agreements of this sort must be honored. Indeed they should be improved upon so that the frightfulness of war can be better and more workably held in check. All men, especially government officials and experts in these matters, are bound to do everything they can to effect these improvements. Moreover, it seems right that laws make humane provisions for the case of those who for reasons of conscience refuse to bear arms, provided however, that they agree to serve the human community in some other way. Certainly, war has not been rooted out of human affairs. As long as the danger of war remains and there is no competent and sufficiently powerful authority at the international level, governments cannot be denied the right to legitimate defense once every means of peaceful settlement has been exhausted. State authorities and others who share public responsibility have the duty to conduct such grave matters soberly and to protect the welfare of the people entrusted to their care. But it is one thing to undertake military action for the just defense of the people, and something else again to seek the subjugation of other nations. Nor, by the same token, does the mere fact that war has unhappily begun mean that all is fair between the warring parties. Those too who devote themselves to the military service of their country should regard themselves as the agents of security and freedom of peoples. As long as they fulfill this role properly, they are making a genuine contribution to the establishment of peace.