ABSTRACT 

The story of the intercession of Pope Leo XIII with the Negus of Ethiopia, Menelik II, for the liberation of the prisoners who fell in Adua in 1896 becomes an opportunity for a reflection on the contribution of Vatican diplomacy, and of the papal magisterium in particular, in context of the birth of international law as an autonomous science. The look at the pontificate of Leo XIII, a pontiff between the Restoration and modernity, at his complex personality, also extends to the peculiar situation of the busy Kingdom of Italy at the end of the nineteenth century. On the internal front, the rift with the Catholic Church must be managed and, on the external front, the uncertain beginning of the colonial experience must be regulated. In this historical framework in which aspirations, disappointments, fears, conflicts, misunderstandings and hopes of the young united Italy are mixed, the figure of Pope Pecci stands out, whose contribution to the ecumenical cause and that of peace between peoples is also transversally recognized by his political opponents. The small cameo that sees him as the protagonist of the intercession to the emperor of Ethiopia for the freedom of the Italian prisoners, defeated in Adua, in the name only of his moral, as the father of Christians, is reason to evaluate the positive contribution of the diplomatic activity of the Saint Seat, as well as of the principles of the Social Doctrine of the Church of which the Leonine pontificate was the driving force, in the dialogue between the Nations between the 19th and 20th centuries.