This article examines the status of just war within contemporary Catholic social doctrine. Special reference is made to just war as it is discussed by Pope Francis in Fratelli tutti. In its first part, the article considers how Thomas Aquinas set the problem of war within the wider horizon of peace understood as a fruit of charity. It then shows how Pope Francis approaches war from very much the same perspective. Just as St. Thomas elucidates how caritas qua friendship should inform all of our societal relations, including within the international sphere, similarly, in Fratelli tutti, Pope Francis urges us to observe fraternity in all areas of our life, interpersonal and social; even nations should adhere to this pattern in their relations with each other. Both St. Thomas and Francis recognize, however, that we can, and often do, fail in this regard. War is the most egregious manifestation of this failure. It is, however, in their respective treatments of war that St. Thomas and Francis seem to part ways; the first affirms the possibility of just war, while the second denies it. Whether this divergence is real or only apparent is examined in the article's second part and its conclusion.