POD: 1914
1914: Mexico's dictatorial Huerta regime is not overthrown by Venustiano Carranza.
1917: Huerta decides to join with Germany, to declare war on the United States, after receiving the Zimmerman telegram.
1918-1920: The United States and Mexico are at war. The American army led by "Black Jack" Pershing slogs through Mexico, before overthrowing Huerta. War casualties approach the World War I total for the European countries, nearly 3% on each side.
1921: Mexico is formally absorbed as a protectorate of the U.S.
At this time, the combined population of Amer-xico is approximately 116 million with 40 million Catholics.
1930's: The New Deal and the "make-work" programs of the depression extend into massive development of Mexico, leading to a demographic shift, as Mexico starts to rapidly grow in population. At the same time, the improvements in Mexico, and the general ruthlessness of companies like United Fruit, lead to large amounts of immigration from Guatemala and Costa Rica into southern Mexico. The "Banana Diaspora" as it was called lead to an even higher amount of catholics in Southern Mexico, now receiving the benefits of the new deal.
1940's to current: The catholics out breed the protestant population of the U.S., and the early improvements to Mexico, and related immigration laws towards latin American countries, pressed for by Mexican representatives and senators lead to a Catholic plurality.
2010: Cenusu data reveals that the U.S - Mexico has a population of 490 million people, with 265 million Roman Catholics. [54.0% Roman Catholic]
The old U.S. has approximately 300 million people with 33% identifying as Roman Catholic.
The old Mexico has approximately 190 million people with 86% identifying as Roman Catholic. [Mexico right now identifies as 88% R.C. according to wikipedia]