The First World War – a total war because not only military but also economic, social and geopolitical – has involved great tragedies, the disappearance of the Empires and of the great Kingdoms, the general impoverishment of the weak classes of society, that is of those who most they have borne the burden, and designed a new political map in Europe. No less serious were the social, political and demographic consequences, notably dramatic in themselves and in the projections on a future, marked by the void left by the over nine million deaths. This war can rightly be categorized as a great defeat of humanity, which through the registered upheavals also generated the premise for World War II. Moreover, it can also be considered in some way as a defeat of the policy pursued by the Holy See which failed, despite the Pope’s appeal and the commitment of Vatican diplomacy, to stop it. However, two positive factors can be noted, so to speak: the first is that the war became part of and supported the recently begun process of bringing Catholics closer to national politics, marking a turning point in overcoming the Risorgimento period; the second is that among the outcomes of the conflict there was also a return to faith and, at the same time, a progressive transformation of the Catholic attitude towards the themes of peace and war.