This article aims to determine the contribution of Pius XII to the development of Catholic doctrine on the problem of war and peace, which has undergone a noticeable transformation during its existence. It gradually switched from the approval of the armed confict under certain circumstances to pacifsm, which was confrmed by the decisions of the Second Vatican Council (1962-1965). In this regard, the interesting question is how the last “preconciliar” pope Pius XII saw the problem of the allowability of war. His pontifcate witnessed the Second World War, he regularly called for an end to the bloodshed and, afer the end of the confict, he continued to develop the idea of establishing and maintaining peace throughout the globe. According to Pius XII, the principal tasks of governments were to search compromise in crisis situations and to prevent military clashes in the future. This could be facilitated by the Catholic teaching and the idea of establishing a Christian order throughout the world. The authors base their conclusions on the analysis of Pius XII’s speeches and encyclicals and demonstrate that the Pope did not support the war and, moreover, did not call for it, but on the contrary, he advocated dialogue and rejection of war as a way to resolve disputes in the circumstances of growing international instability and confrontation between the two opposing blocs. Thus, his views were a logical continuation of the peaceloving ideas of his predecessors. The Pope concerned with nuclear weapons, which appeared during his pontifcate and could give rise to an unprecedented confict and plunge the world into a worldwide catastrophe, paid special attention to the issues of limiting scientifc research. Finding himself in more difcult situation than the previous Roman pontifs, Pius XII more decisively condemned war, even the “just” one. The ideological foundation of the decisions of the Second Vatican Council was fnally built on these concepts.