The establishment of Mao Zedong's communist regime in 1949 put these early advances on hold and led to the persecution of thousands of clergy and faithful in China. The losses in the following years were considerable. Some of the Catholic hierarchy, including the archbishops of Nanking and Peking, left the mainland and eventually made their way to Taiwan. In 1951, the Papal Nuncio, Archbishop Riberi, was expelled, as were many foreign missionaries, who were accused of acting as agents of imperialist forces. Hundreds more Catholic clergy experienced increased supervision, frequent arrests and torture, and Catholic laypeople were under tremendous pressure to renounce their faith. The Holy See reacted with several encyclicals and apostolic letters, "Cupimus Imprimis", "Ad Apostolorum principis", and "Ad Sinarum gentem" (1954).