44 Since the Church is the safe and sure guide to conscience, for to her safe-keeping alone there has been confided the doctrines and the promise of the assistance of Christ, she is able not only to bring about at the present hour a peace that is truly the peace of Christ, but can, better than any other agency which We know of, contribute greatly to the securing of the same peace for the future, to the making impossible of war in the future. For the Church teaches (she alone has been given by God the mandate and the right to teach with authority) that not only our acts as individuals but also as groups and as nations must conform to the eternal law of God. In fact, it is much more important that the acts of a nation follow God's law, since on the nation rests a much greater responsibility for the consequences of its acts than on the individual.
Source: Ubi Arcano Dei Consilio (Vatican.va)