This chapter provides a critical examination of the just war tradition in jus ad bellum from a fusion of the practice of ancient times, early Roman law, and Christian and secular teachings. The Just War theory has a varied and diverse background. It deals with the justification of how and why wars are fought. It has several commonly recognised elements, which are traditionally divided into two basic categories that deal with the two basic fundamental issues regarding the legitimacy of use of force. The Just War theory in the modern concept was refined and formalised by Christian theologians and philosophers beginning with St. Augustine in the fifth century, even though the tradition of recognising constraints on the conduct of war is much older. A great impetus was given to the Just War theory by St. Thomas Aquinas who was the second major Christian thinker to deal with the issue of war.