Over the course of the First World War, even given its prolongation and the increasing number of victims, the acceptance of obedience to political power changed into an open justification of war in religious terms: war as a sacrifice, as a crusade. However, this sanctification of the war, twisted for the purposes of a national religious discourse, was in sharp contrast to the action of the Pontiff who, in August 1917, replied with a famous Note to the leaders of the belligerent peoples, in which he took a precise position against the ongoing conflict.
One of the peculiar aspects of the Great War was certainly that of the broad approval for the conflict, its "ideals", and the goals of the war proclaimed by the governments and hailed by the propaganda. This approval contrasts violently with the course of the conflict, the enormity of loss that it implied, and the increasingly evident lack of sense in so much fighting and dying.
The explanation for this apparent contradiction must also take into account the ecclesiastical apparatus, which played an important role in influencing public opinion in each of the warring nations. From the time war was declared, the clergy and large organizations took part in the mobilization for war along with the parliaments and the vast majority of national political classes. This was readily understandable in those countries, such as Germany, Great Britain, Russia, and Serbia in which the national churches were strongly tied to the state. The situation for Catholics was more complicated in France or Austria, for example, where loyalty was expected to lay more with the Roman Pontiff than with the State. But the reasons for patriotism, and readiness to join the pro-war climate, where - clearly in the French case – Catholics did not wish to be instructed in patriotism by a fiercely anti-clerical ruling class, got the upper hand. There were those among the Catholics who wanted a theological element present in Catholic and Christian teaching to be the paladin of intervention into the war: the just war.
This doctrine taught that the faithful-citizens should accept the reasons for the war proposed by the holders of political power, who alone had knowledge of all the elements that justified a war conducted on proper grounds. Once the conflict began Catholics would have to adapt and accept the explanations given by the political power and commit themselves to following the orders of civil and military authorities. There were many reasons for declaring a just war, and these took different forms in individual cases: French Catholics could be informed that they were fighting to defeat their arch-enemies the Lutherans, while for Austrian or German Catholics the goal of fighting was to eliminate the risk that if their country were defeated, it would be overwhelmed by the dangerous "ideas of 1889": materialism and secularism, and so forth and so on. Once a national church had entrusted its own country with the task of achieving the restoration of peaceful Christian society through the war, Catholics were obliged to obey, and proved to be exemplary soldiers and patriots.
Over the course of the conflict, even given its prolongation and the increasing number of victims, the acceptance of obedience to political power changed into an open justification of war in religious terms: war as a sacrifice, as a crusade. A very influential secular intellectual, like D'Annunzio, made skilful use of such reasoning from the start of the intervention campaign of May 1915. This sanctification of the war, twisted for the purposes of a national religious discourse, was in sharp contrast to the action of the Pontiff. The Pope had had a very "hard" life since the outbreak of the conflict. He was the head of the universal Roman church, but also the head of the Italian church, as well as being an Italian himself. The Italian church had had more than a few disagreements with the unified state since the conquest of Rome in 1870 and, at the time of the debate on the intervention and in the months that followed, it had to deal with a state ready to resort to censorship and incrimination of priests and bishops who dared to even whisper against the seemingly firm national unity that supported the war. The church was frequently accused of being lukewarm and "leaning towards Austria". And Benedict XV, born Giacomo della Chiesa, and pope since September 1914, had to deal with a church which, to consolidate its national legitimacy, had more or less openly come out in favour of the war. We might think of figures like Father Agostino Gemelli, future founder of the Catholic University, confidant of the extremely religious General Cadorna. Gemelli was among the first to define the typical psychological traits of obedience of the conscript army. Then there was Father Giovanni Semeria, who was among the most fervent advocates of the sanctification of the war. Benedict XV had to intervene frequently to censor priests from twisting the prayers for peace, so strongly recommended by the Pontiff, into nationalistic pleas: prayers for peace were being transformed into prayers for victory.
The famous letter sent by the pope to the rulers of the belligerent countries after a tormented silence, on 1 August 1917, contains two very important aspects: first, a passionate invitation to start peace negotiations, starting with the principle of a renunciation of all territorial conquests achieved so far and a return to the status quo. Benedict XV set out a programme that anticipated President Wilson's fifteen points the following year, which included freedom of the seas, mutual disarmament, and the recognition of the national aspirations of peoples living in the empires. Moreover, the most famous phrase of the letter stating: "every day the war seems to be a useless slaughter" is of great theological significance. If the war had become nothing more than a useless slaughter, the very reasoning that legitimized the use of war had failed. If the supreme authority of the Catholic Church proclaimed the uselessness of war, war could no longer help restore the proper Christian order of collective life that the enemy had violated. It appears anti-historical to accuse - as the military commands did - Benedict XV, along with the Socialists, of having been the inspiration of the "rebellion" against the war that a few months later would become strengthened by the route at Caporetto. In any event, the Pope's letter put a rather large dent in the consistency of the ecclesiastical argument that sought to legitimize the war.