|The following is based on the Fallout Bible and is not considered canon.|
Mexico was under significant pressure from the U.S. to solve its severe political instabilities as well as widespread pollution, which also caused problems in the U.S. The U.S. saw Mexico's inability to conform to their wishes as a threat to national security, prompting an invasion to secure the country's refineries, as well as to ensure that the already scarce resources safely crossed the U.S.-Mexican border. The United States attempted to launch an occupation/annexation operation, but the final decision of this, as well as the economic and political outcomes of the invasion, is unknown.
The American invasion of Mexico is mentioned only in the Fallout Bible.