The Mexican Revolution was a momentous challenge for the Holy See in the Benedict XV’ pontificate, that faced the persecution of the Catholic Church led by the Constitutionalist forces of Venustiano Carranza. After the Catholic Hierarchy was accused of participating in the 1913 coup d’état against President-in-Office Francisco I. Madero, the promulgation of the Queretaro’s Constitution (February 5th, 1917) represented the heaviest attack on the Catholic Church from Mexican law since the independence. Following the closure of the Apostolic Delegation in Mexico and the banishment of almost all Mexican bishops in 1914, the Vatican Diplomacy tried to involve the US government in an effort to bring Carranza to negotiate with the Church. However, relations between Church and State became better at the end of the pontificate, when the Apostolic Delegation in Mexico reopened. Despite the marginality of Mexico with regard to the First World War, there were interesting connections between the Church-State conflict in revolutionary Mexico and the European war, due to the role played by Vatican Diplomacy as a “transmission belt”. As a matter of fact, during and after the World War, the Vatican sought unsuccessfully to make the World Powers aware of the necessity to improve the religious situation in Mexico.