The Cristero War also known as the La Cristiada (1926 to 1929) was a revolutionary uprising and counter-revolution against the Mexican government during that time. The original rebellion was set off by the persecution of Roman Catholics and a ban on their public religious practices. More specifically, the strict enforcement of the anti-clerical provisions by atheist and former Mexican President Plutarco Elias Calles through the Mexican Constitution of 1917 along with the further expansion of anti-clerical laws exacerbated the conflicts. The rebellion is particularly known for the Mexican women and children who assisted the rebels in smuggling guns and ammunition in secret, and certain Mexican priests who were tortured and murdered in public, as later canonized by Blessed Pope John Paul II.
After a period of peaceful resistance by select citizens, a number of skirmishes took place in 1926 while the formal violent rebellions began on 2 January 1927, with the rebels who called themselves Cristeros, invoking the name of Jesus Christ under the title of "Cristo Rey" or Christ the King. The rebellion eventually ended by diplomatic means brokered by the then United States Ambassador to Mexico, Dwight Whitney Morrow along with the assistance of the Knights of Columbus.
The rebellions attracted the attention of Pope Pius XI who later, under the papal bull Firmissimam Constantiam of 28 March 1937, denounced the extreme anti-clericalism in Mexico. Pope Pius XI expressed his opposition to the "impious and corruptive school" (p. 22) and granting papal support for Catholic Action in Mexico for the third consecutive time with the use of plenary indulgence, along with the papal bulls Iniquis Afflictisque (Of the persecution of the Church in Mexico) and Acerba Animi.