Out of this tension and search for answers in Christian thought, emerged the Just War Theory of St. Augustine of Hippo.
Augustine continued in the Christian tradition of believing all war to be intrinsically evil. He unequivocally condemned those who desired, sought, or enjoyed war, and made it clear that to engage in a just war is to engage in war by force of necessity. In Book 4, chapter 14 of his magnum opus, The City of God, he says “to carry on war and extend a kingdom over wholly subdued nations seems to bad men to be felicity, to good men necessity .” Later, in Book 9, chapter 7 of the same work, he states the same even more clearly: “But, say they, the wise man will wage just wars. As if he would not all the rather lament the necessity of just wars, if he remembers that he is a man; for if they were not just he would not wage them, and would therefore be delivered from all wars. For it is the wrongdoing of the opposing party which compels the wise man to wage just wars ”.
In all of this, Augustine is not far from and is, in fact, probably drawing upon the ideas of Cicero and the author of Deuteronomy. He goes further than either of them, however, in his condemnation of war itself and in his refusal to allow that the aggressor may be just. For Augustine, war is never a good but only a lesser of evils, and the one who causes the war is always unjust.
Immanuel Kant, one of the greatest philosophers in the history of Western Civilization, was also an opponent of