Justice in war has become an influential perspective. In particular, the moral implication highlights the fundamental importance of the “Just War” theory. The principle was first established in ancient Rome in 106-43 BC by the Roman philosopher Cicero, who stated that "no war is considered just unless it is preceded by an official demand for satisfaction or warning and is a formal declaration has been made." (Cicero, 1913, p.38-39). Therefore, it is a precedent that a war is established based on the principle of justice. The theory was further coined by the Roman Christian philosopher, Augustine of Hippo (345-430 BC) and later carried forward by Thomas Aquinas (1274 BC). The principle was used to delve into the question of when it was permissible to wage war and the conduct of war. Both Christian and Greek philosophers had conflicts about when and how to fight in a war. Therefore, the moral goal for both philosophers was to establish peace. During this period, Thomas Aquinas became one of the most influential philosophers on the principle of just war. He argues that for a war to be just, it must meet three criteria: "(1), the war had to be waged not privately but under the authority of a prince, (2) there had to be a just cause for the war", (3) it was necessary to have the right intention to promote good and avoid evil', (Dinstein, 2005, p.64). Aquinas emphasizes that the principle of jus ad bellum focuses on the moral justification of war. The moral conduct of war, however, is implemented through the principle of jus in Bello. Therefore, it can be assessed that just war theory implements a set of rules to justify military war. Just war theory now: The 21st century has marked an era for the just war principle. The theory of justice and war allowed p...... middle of paper...... hand, the principle is still very useful and is mentioned in the global political and social debate. It is notable that Richard Falk, a critic of Western wars, argues that just war theory “is a vital source of modern international law that governs the use of force and focuses attention on the causes, means and ends of war.” (Shaw, 2005, p.133). It can be recognized that the morality of war still remains urgently central to political discussions around the world. In recent years, just war theory has sought to respond to major challenges related to the outbreak of the Iraq war in 2003. It can be assessed that the Iraq war has distorted the existence of weapons of mass destruction into a challenging theory. Therefore, this thesis will elaborate the theories challenged by the Iraq war in relation to the use of weapons of mass destruction.