From Pope Pius XI’s encyclical Quas Primas (1925):
If the kingdom of Christ, then, receives, as it should, all nations under its sway, there seems no reason why we should despair of seeing that peace which the King of Peace came to bring on earth—he who came to reconcile all things, who came not to be ministered unto but to minister, who, though Lord of all, gave himself to us as a model of humility, and with his principal law united the precept of charity; who said also: “My yoke is sweet and my burden light.” Oh, what happiness would be Ours if all men, individuals, families, and nations, would but let themselves be governed by Christ! “Then at length,” to use the words addressed by our predecessor, Pope Leo XIII, twenty-five years ago to the bishops of the Universal Church, “then at length will many evils be cured; then will the law regain its former authority; peace with all its blessings be restored. Men will sheathe their swords and lay down their arms when all freely acknowledge and obey the authority of Christ, and every tongue confesses that the Lord Jesus Christ is in the glory of God the Father.”33
33 Enc. Annum Sanctum, May 25, 1899.
— Quas Primas 20