Mankind must put an end to war, or war will put an end to mankind. (John F. Kennedy).
He who joyfully marches to music in rank and file has already earned my contempt. He has been given a large brain by mistake, since for him the spinal cord would fully suffice. This disgrace to civilization should be done away with at once. Heroism at command, senseless brutality, deplorable love-of-country stance, how violently I hate all this, how despicable an ignorable war is; I would rather be torn to shreds than be a part of so base an action! It is my conviction that killing under the cloak of war is nothing but an act of murder. (Albert Einstein)
It is our clear duty, therefore, to strain every muscle in working for the time when all war can be completely outlawed by international consent. This goal undoubtedly requires the establishment of some universal public authority acknowledged as such by all and endowed with the power to safeguard on the behalf of all, security, regard for justice, and respect for rights. But before this hoped for authority can be set up, the highest existing international centres must devote themselves vigorously to the pursuit of better means for obtaining common security. Since peace must be born of mutual trust between nations and not be imposed on them through a fear of the available weapons, everyone must labour to put an end at last to the arms race, and to make a true beginning of disarmament, not unilaterally indeed, but proceeding at an equal pace according to agreement, and backed up by true and workable safeguards. In the meantime, efforts which have already been made and are still underway to eliminate the danger of war are not to be underrated. On the contrary, support should be given to the good will of the very many leaders who work hard to do away with war, which they abominate. These men, although burdened by the extremely weighty preoccupations of their high office, are nonetheless moved by the very grave peacemaking task to which they are bound, even if they cannot ignore the complexity of matters as they stand. (Second Vatican Council. Gaudium et Spes, 1965)
You cannot simultaneously prevent and prepare for war (Albert Einstein).