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Persecutions of the
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 consecreate 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 5000 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 Mexican 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, secularising 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.
June 3, 1933 he issued the encyclical Dilectissima Nobis, in which he described the expropriation of all Church buildings, episcopal residences, parish houses, seminaries and monasteries. By law, they are now property of the Spanish State, to which the Church will have to pay rent and taxes in order to continuously use these properties. "Thus the Catholic Church is compelled to pay taxes on what was violently taken from her" Religious vestments, liturgical instruments, statues, pictures, vases, gems and similar objects necessary for worship were expropriated as well. 
The churches were not spared in the expropriation. Numerous churches and temples were destroyed by burning, after they were nationalized. Jesuits were prohibited from teaching. Private Catholic schools from Religious Orders and Congregations were expropriated, without regard to the free will of founders and benefactors. The purpose was to create solely secular schools there instead.
The Civil War in Spain started in 1936, during which thousands of churches were destroyed, eleven bishops and 7000 clergy and Religious assassinated  After that, Spanish Catholics largely supported Franco and the Nationalist forces in the Spanish Civil War of 1936 – 1939. It is estimated that in the course of the Red Terror, 6,832 members of the Catholic clergy were killed.
Another source breaks down the figures as follows: Some 283 women religious were killed. Some of them were badly tortured.  13 bishops were killed from the dioceses of Siguenza, Lleida, Cuenca, Barbastro Segorbe, Jaen, Ciudad Real, Almeria, Guadix, Barcelona, Teruel and the auxiliary of Tarragona.
Aware of the dangers, they all decided to remain in their cities. I cannot go, only here is my responsibility, whatever may happen, so the Bishop of Cuenca  In addition 4172 diocesan priests, 2364 monks and friars, among them 259 Clarentians, 226 Franciscans, 204 Piarists, 176 Brothers of Mary, 165 Christian Brothers, 155 Augustinians, 132 Dominicans, and 114 Jesuits were killed. In some dioceses, the number of secular priests killed are overwhelming:
- In Barbastro 123 of 140 priests were killed.  about 88 percent of the secular clergy were murdered, 66 percent
- In Lleida, 270 of 410 priests were killed.  about 62 percent
- In Tortosa, 44 percent of the secular priests were killed.
- In Toledo 286 of 600 priests priests were killed. 
- In the dioceses of Malaga, Menorca andSegorbe, about half of the priests were killed"
- Persecutions of the Catholic Church and Pius XII
- Persecution of Christians in the Soviet Union
- Persecution of Christians in Warsaw Pact countries
- ↑ Fontenelle, 164
- ↑ (Hansjakob Stehle, Die Ostpolitik des Vatikans, Piper, München, 1975, p.139-141
- ↑ Riasanovsky 617
- ↑ 4.0 4.1 Riasanovsky 634
- ↑ 5.0 5.1 5.2 Franzen 398
- ↑ "Homily of Pope John Paul II: Canonization of 27 New Saints, Sunday, 21 May 2000".
- ↑ Dilectissima Nobis, 9-10
- ↑ Dilectissima Nobis, 12
- ↑ Dilectissima Nobis, 21
- ↑ Franzen 397
- ↑ ref name="cueva355"
- ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 Jedin 617
- ↑ Beevor 2006, pp. ???
- ↑ 14.0 14.1 de la Cueva 1998, p. 355