Peace does not mean merely the absence of war, nor is it limited to a mere balance of opposing forces, nor does it arise from oppressive domination; rather, it is correctly and precisely referred to as the "work of justice." (…) Because people are sinful, the danger of war threatens them and will continue to do so until the coming of Christ. (…) Recent wars have caused immense material and moral damage to our world, and even now, day by day, war continues its destructive work in some parts of the Earth. (…)
In the face of this humiliating condition of humanity, the council wants to primarily remind us of the enduring validity of natural international law and its general principles. The conscience of humanity proclaims these principles firmly and with growing emphasis. Actions that knowingly contradict them, as well as orders by which such acts are commanded, are criminal, and blind obedience cannot excuse those who comply. Among these actions, the systematic and deliberate extermination of entire populations, nations, or ethnic minorities must be unequivocally condemned as a heinous crime. Conversely, the attitude of people who are not afraid to openly resist such orders deserves the highest recognition.
War has not yet been eradicated from the human world. As long as the danger of war exists and there is no corresponding international authority equipped with sufficient power, governments cannot be denied the right to just defense, but only after exhausting all means of peaceful negotiation. Heads of state and all those who participate in the responsibility for the state have the duty to protect the welfare of the people entrusted to them, and they must take this task with the utmost seriousness. However, it is one thing to take up arms in just defense, and another to seek to subjugate other nations. (…)
Those who serve their country in the military should consider themselves as servants of the security and freedom of nations. When they properly fulfill this task, they genuinely contribute to the strengthening of peace.
Any military action aimed recklessly at the destruction of entire cities or vast territories along with their inhabitants is a crime against God and humanity, which must be condemned with determination and without hesitation.
(Pastoral Constitution Gaudium et spes, paragraphs 78.79.80; December 1965)
