Based on the analysis of the works of medieval Christian thinkers, the article highlights the principles of the Christian doctrine of „just war”, which was formed during the Middle Ages and did not undergo significant changes until the Second Vatican Council (1963-1965). The urgency of the topic lies in the fact that many armed conflicts are now religious in nature, including the latest Russian-Ukrainian war, which is being waged under the slogan of the „Russian world” ideology, developed by Moscow Patriarch Kirill. Such and similar consecration of wars and armed conflicts by religious organizations forces us to turn to political and legal concepts, which seem to have long since disappeared, but they now, as it turns out, acquire a new meaning. The article reveals the purpose of the study, its methods and the state of study in the literature. The main part of the article shows that, although the concept of „just war” dates back to antiquity, the architects of the doctrine of just war were medieval Christian authors – Ambrose of Milan, Aurelius Augustine, Thomas Aquinas, Francisco de Vitoria. The article considers the contribution of each of these authors to the development of the doctrine, emphasizes that, in fact, the concept of „doctrine” should be attributed to the creative legacy of Thomas Aquinas in the concept of „just war”, and noted that this doctrine became the basis of Catholic doctrine of war. which has remained unchanged for many centuries. Later scholastics, and above all Francisco de Vitoria, supplemented the doctrine of just war with their research, without affecting its foundations developed by Thomas Aquinas. However, it is noted that in modern warfare, the concept of „justice” is distorted, the understanding of just war in the Christian religious and ethical tradition has been forgotten, and some church leaders even suggest abandoning the doctrine of just war. The author believes that everything needs to be redesigned, raising the whole layer of works on the theme of just war, starting with ancient and medieval authors. The article concludes with conclusions and formulations of prospects for further research in this area.