The combination of terms of religion, war, and international relations might at first conjure associations with past historic events such as the Thirty Years’ War. But they could also trigger associations with recent examples, such as the Russian Orthodox Church addressing the United Nations Security Council during the ongoing Ukraine War or hundreds of Jewish scholars and religious representatives appealing to Pope Francis in an open letter during the ongoing Gaza War. How does the field of study of International Relations (IR) engage with questions of religion and war? This paper addresses the long neglect the field experienced in terms of including religion in its research agenda and presents the increase of interest and approaches which has emerged in the last two decades. By including work from closely related disciplines, such as history and religious studies, it addresses the potential of interdisciplinary work and identifies points of departure for future IR research. The literature is classified into four categories: “Religion, War, and History”; “Religion, War, and Political Science”; “Religion, War, and Theology”; “Religion and Peace”. The contributions of each of these categories are loosely structured according to the Just War criteria of jus ad bellum, jus in bello, and jus post bellum. Overall, the state of the art indicates that the juxtaposition of religion, war, and IR lends itself extremely well to interdisciplinary research and can contribute valuable insight into changes in international society.