This paper examines the emergence of a Catholic doctrine of international order during the early years of the Second World War, focusing on the teaching of Pope Pius XII and its reception within a transatlantic Catholic intellectual network. Beginning with the encyclical Summi Pontificatus (1939) and the Christmas messages of 1939-1941, the paper argues that the Holy See articulated a conception of international society grounded in natural law, the dignity of the human person, and the juridical equality of sovereign states. Within this framework, national self-determination, minority rights, disarmament, and international cooperation were presented as necessary conditions for a just and durable peace. The paper further shows how this papal doctrine was interpreted and systematized by Catholic commentators in both Europe and the United States, including Guido Gonella, Edward A. Wright, Francis J. Watt, and John M. Beales. Their works translated papal pronouncements into a coherent theory of international order centred on a morally regulated society of states. This intellectual exchange formed part of a broader transatlantic dialogue on post-war reconstruction. Finally, the paper situates this Catholic reflection within the parallel diplomatic dialogue between the Vatican and the Roosevelt administration. American wartime planning—articulated through the Four Freedoms, the Atlantic Charter, and discussions transmitted to the Holy See through Myron Taylor—frequently emphasized principles that closely paralleled papal teaching on justice, human dignity, and international cooperation. By reconstructing this convergence of papal doctrine, Catholic intellectual commentary, and wartime diplomacy, the paper highlights the role of Catholic thought in shaping mid-twentieth-century debates about the moral foundations of international order.