This paper deals with the development of the doctrine and theory of just war throughout history, which is one of the most influential theories regarding the morality of warfare. War has existed since times immemorial, and efforts have been always made to justify it through various rules of engagement and rules of conduct during war. The foundation of the doctrine of jus ad bellum (the right to wage war) was laid in Roman law, and numerous later scholars based their theories on just war on this foundation. The doctrine evolved from Roman law to the modern era, and almost all proponents of the theory of just war agree that a just war is tied to the conditions of just cause, right intention, and legitimate authority. In the modern era there is a tendency towards the humanization of warfare and the establishment of certain rules that warring parties should adhere to. International humanitarian law and the law of armed conflict regulate numerous rules that reflect the principles advocated by theorists of just war. Under modern international law, war is prohibited according to the Charter of the United Nations, except in cases of self-defense or when the use of force is authorized by the Security Council in response to a threat to peace, a breach of peace, or an act of aggression.