This study examines the traditional theory of just war in the light of modern principles of international law relating to the prohibition on the use of force repeatedly stressed by UNGA (United Nations General Assembly) resolutions and accepted by the ICJ (International Court of Justice). The author expresses doubts as to whether actions by some permanent members of the Security Council starting from September 1996 until April 2003, in the Balkans and the Persian Gulf, are legitimate under the just war theory, or any other rules of international law, and analyses in detail the claims made by the allied powers to justify their actions. The book also examines the significance of the transformation in the limitation and prohibition of the use of force in the contemporary legal system, by studying the origin of those tenets and their reflection in both the national laws of individual states and the international laws of armed conflict.