Mexico was under severe pressure from the US to solve its severe political instabilities, widespread pollution which also caused problems in the US, and the many economic groups destabilizing the country. The US saw Mexico's inability to conform to US wishes as a threat to national security, prompting an invasion in order to secure the nation's refineries, as well 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 is, as well as the economic and political outcomes of the invasion, is unknown.
Edit