Template:20th century persecutions of the Catholic Church Terrible Triangle was a term used by Pope Pius XI for the simultaneous persecution of Christians in general and the Catholic Church in particular in three countries: the Soviet Union, Mexico, and Spain. These events are said to have influenced his position on Communism throughout his pontificate. Pope Pius XI labeled the failure to protest and react in Europe and the United States as a Conspiracy of Silence.
Pius XI was horrified by Communist persecution of Christians in the Soviet Union, but he mandated Berlin Nuncio Eugenio Pacelli to work secretly on diplomatic arrangements between the Vatican and the Soviet Union. Pacelli in the name of the pope negotiated food shipments for Russia, where the Church was persecuted. He met with Soviet representatives including Foreign Minister Georgi Chicherin, who rejected any kind of religious education, the ordination of priests and bishops, but offered agreements without the points vital to the Vatican. Despite Vatican pessimism and a lack of visible progress, Pacelli continued the secret negotiations until Pius XI ordered them to be discontinued in 1927 because they generated no results and were dangerous to the Church if made public.
The harsh persecution, short of total annihilation, of the clergy, monks, and nuns and other people associated with the Church  continued well into the thirties. In addition to executing and exiling many clerics, monks and laymen, the confiscating of Church implements "for victims of famine" and the closing of churches were common.  Yet according to an official report based on the census of 1936, some 55% of Soviet citizens identified themselves openly as religious, while others possibly concealed their belief. In 1937 the Pope issued the encyclical Divini Redemptoris, which was a condemnation of Communism and the Soviet regime." He did name a French Jesuit to go to the USSR and consecrate in secret Roman Catholic bishops. It was a failure, as most of them ended up in gulags or were otherwise killed by the communist regime.
During the pontificate of Pius XI, the Catholic Church was subjected to extreme persecutions in Mexico, which resulted in the death of over 5,000 priests, bishops and religious.  In the state of Tabasco the Church was in effect outlawed altogether. In his encyclical Iniquis Afflictisque from November 18, 1926, Pope Pius protested against the slaughter and persecution. The United States of America intervened in 1929 and moderated an agreement. The persecutions resumed in 1931. Pius XI condemned the Mexican government again in his 1932 encyclical Acerba Animi. Problems continued with reduced hostilities until 1940, when in the new pontificate of Pope Pius XII President Manuel Ávila Camacho returned the Mexican churches to the Catholic Church.
One symbol of the massive Church persecution was Miguel Agustín Pro Juárez, S.J. (January 13, 1891 – November 23, 1927), a Mexican Roman Catholic Jesuit priest. He was executed during the persecution of the Catholic Church under the presidency of Plutarco Elías Calles after trumped up charges of involvement in an assassination attempt against former President Álvaro Obregón. Fr. Pro was beatified by John Paul II as a martyr on September 25, 1988. On May 21, 2000, Pope John Paul II canonized a group of 25 saints and martyrs arising from the Cristero War.
The vast majority are Roman Catholic priests who were executed for carrying out their ministry despite the suppression under the anti-clerical laws of Plutarco Elías Calles. Priests who took up arms, however, were excluded from the process. The group of saints share the feast day May 25
The Republican government which had come to power in Spain in 1931 was strongly anti-clerical, secularizing education, prohibiting religious education in the schools, and expelling the Jesuits from the country. On Pentecost 1932, Pope Pius XI protested against these measures and demanded restitution. He asked the Catholics of Spain to fight with all legal means against the injustices.
- Persecutions of the Catholic Church and Pius XII
- Persecution of Christians in the Soviet Union
- Persecution of Christians in Warsaw Pact countries