*Musing*
In 1926 the "Calles Laws" are passed and the outcry against the anti-catholic laws in Mexico are even larger than OTL. It quickly escalates into a violent rebellion, but this time many militias and federal soldiers refuse to fight against the church-led rebels. The Catholic rebels receive financial support from catholics in the US. The Federal government orders more draconian methods in order to destroy the rebel forces, but that just creates more sympathy and portions of the army defect. It comes to a head in 1928 when a Catholic army marches on Mexico City and seizes the government, ending the war. The American Ambassador recognizes the government and offers American assistance in rewriting the constitution and repairing the economic damage in exchange for an agreement on allowing American oil exploration.
The Catholic schools are reopened nationally and the property of the impoverished is no longer being seized without payment by the government. The initial American funds (not too many, since the great depression starts soon) and the drilling for oil help the economy and create the Mexican Miracle, a sustained period of robust economic growth, in spite of the global recession.
Mexico sends support to the Nationalists during the Spanish Civil War unlike OTL. After the war ends, relations between Spain and Mexico will remain friendly, in spite of some discomfort among the Mexican government over Franco's despotic nature. Mexico also fights in the 2nd World War.
After the World War, Mexico becomes a founding member of NATO, with the interest of checking the spread of communism. Trade with the United States increases and Industry continues to be developed. In 1954, Mexico is supplied with an experimental reactor by Canada and heavy water from the United States to do research into a nuclear power program.
In 1961, Mexico assists the CIA in training for the ill fated Bay of Pigs invasion. As a result, Soviet missiles are installed in Cuba and during the Cuban Missile Crisis, Mexico is threatened with annihilation right alongside the United States. The state resolves to become a nuclear power in 1963.
Using a smallish group of scientists, assistance from the United States (Until 1968) and the reactor they were supplied with, Mexico manages to construct a Little Boy-design bomb and tests in 1973, becoming the seventh nation to do so. They build an arsenal of six bombs before ceasing production.