Who were the "Cristeros"?
The Cristeros were rebels in Mexico who stood up against the radical secularism of the Mexican Government in a civil war that lasted from 1926-29.
Mexico's turbulent 19th Century politics led to the 1910-1920 Mexican Revolution, a period of tremendous civil violence in the country that may have resulted in 1-2 million deaths. The 'Liberal' Faction eventually won -- the PRI Party essentially ran Mexico until the 1990s, but it had its own internal doctrinal debates.
President Venustiano Carranza, (1915–20) and Alvaro Obregón (1920–24), did not enforce the anticlerical laws sought by many Mexican secularists. The Calles administration (1924–28) was more doctrinaire and stepped down hard on the Church. This resulted in civil disobedience, and President Plutarco Calles escalated his persecution of the Catholic Church. High-handed brutality by Mexican police soon sparked a major rebellion.
The Cristeros were mostly peasants from the central provinces of Mexico. Many were veterans of the Civil War themselves and the Mexican police and Army were soon taking very heavy casualties; and resorted to even heavier repression. About 90,000 people were killed -- about 57,000 police and soldiers and 30,000 Cristeros.
Eventually, the Mexican government negotiated an entente with the Catholic Church, the anti-clerical laws would remain on the books, but would not be enforced while the Church would refrain from active involvement in politics. This situation lasted until 1992, although anti-Catholic death squads made occasional appearances through the 1930s -- there was some similar activity by former Cristeros too.
Interestingly, the Cristeros received aid from overseas from Ireland and the Knights of Columbus in the US. Among the people who raised money for the Mexican government to suppress the conflict were the KKK. Since the end of PRI domination of Mexican politics in 1992, the Catholic Church has recognized 38 Martyrs as candidates for beatification.