<P> A 2017 study found that the just war tradition can be traced as far back as to Ancient Egypt, "demonstrating that just war thought developed beyond the boundaries of Europe and existed many centuries earlier than the advent of Christianity or even the emergence of Greco - Roman doctrine ." </P> <P> In ancient Rome, a "just cause" for war might include the necessity of repelling an invasion, or retaliation for pillaging or a breach of treaty . War was always potentially nefas ("wrong, forbidden"), and risked religious pollution and divine disfavor . A "just war" (bellum iustum) thus required a ritualized declaration by the fetial priests . More broadly, conventions of war and treaty - making were part of the ius gentium, the "law of nations", the customary moral obligations regarded as innate and universal to human beings . The quintessential explanation of Just War theory in the ancient world is found in Cicero's De Officiis, Book 1, sections 1.11. 33--1.13. 41 </P> <P> Christian theory of the Just War begins with Augustine of Hippo and Thomas Aquinas . </P> <P> Augustine of Hippo claimed that, while individuals should not resort immediately to violence, God has given the sword to government for good reason (based upon Romans 13: 4). In Contra Faustum Manichaeum book 22 sections 69--76, Augustine argues that Christians, as part of a government, need not be ashamed of protecting peace and punishing wickedness when forced to do so by a government . Augustine asserted that this was a personal, philosophical stance: "What is here required is not a bodily action, but an inward disposition . The sacred seat of virtue is the heart ." </P>

According to just war theory in order for a war to be justified