Are you sure? A recent poll taken in Mexico indicated a clear majority of Mexicans (a fiercly nationalistic lot themselves) consider the SW United States rightfully belongs to Mexico.
That attitude might not matter much to international institutions such as the catholic church but it sure a heck matters to a large number of Americans, including your's truly.
I hadn't heard that. Can you get me a link?
Of course, the SW USA was Mexico for at least 150 years (depending upon what year you consider that the Spanish claim was made reality, not just as a label on a map but as a fait accompli.) Throughout the 18th century, Catholic missionaries established missions in Texas, New Mexico, California, etc. and the Spanish and later Mexican authorities established towns and cities.
In the early 19th century, the English-speaking Americans established their claims e.g. in Texas precisely through immigration--- at first legal, and then illegal. And the Mexican-American War wrapped up the deal: the illegal immigrants (Anglos) stole the place fair and square.
Now before you flame me up one side and down the other, I'm just citing this as history, not policy. I am not at all an Aztlanista.