45 When, therefore, governments and nations follow in all their activities, whether they be national or international, the dictates of conscience grounded in the teachings, precepts, and example of Jesus Christ, and which are binding on each and every individual, then only can we have faith in one another's word and trust in the peaceful solution of the difficulties and controversies which may grow out of differences in point of view or from clash of interests. An attempt in this direction has already and is now being made; its results, however, are almost negligible and, especially so, as far as they can be said to affect those major questions which divide seriously and serve to arouse nations one against the other. No merely human institution of today can be as successful in devising a set of international laws which will be in harmony with world conditions as the Middle Ages were in the possession of that true League of Nations, Christianity. It cannot be denied that in the Middle Ages this law was often violated; still it always existed as an ideal, according to which one might judge the acts of nations, and a beacon light calling those who had lost their way back to the safe road.
Source: Ubi Arcano Dei Consilio (Vatican.va)