7 One thing is certain today. Since the close of the Great War individuals, the different classes of society, the nations of the earth have not as yet found true peace. They do not enjoy, therefore, that active and fruitful tranquillity which is the aspiration and the need of mankind. This is a sad truth which forces itself upon us from every side. For anyone who, as We do, desires profoundly to study and successfully to apply the means necessary to overcome such evils, it is all-important that he recognize both the fact and the gravity of this state of affairs and attempt beforehand to discover its causes. This duty is imposed upon Us in commanding fashion by the very consciousness which We have of Our Apostolic Office. We cannot but resolve to fulfill that which is so clearly Our duty. This We shall do now by this Our first encyclical, and afterward with all solicitude in the course of Our sacred ministry.
Source: Ubi Arcano Dei Consilio (Vatican.va)