|The following is based on the Fallout Bible and is not considered canon.|
Mexico was under huge pressure from the US to solve its severe political instabilities as well as widespread pollution, which also caused problems in the US. The US saw Mexico's inability to conform to their wishes as a threat to national security, prompting an invasion in order to secure the country's refineries, as well as to ensure that the already scarce resources safely crossed the US-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.