I'll be brave and dare to disagree with you. In the case of the "Zimmermann telegram" during WW I, if you'll recall, Mexico was in the throes of a protracted "revolution," which is simply a euphemism for anarchy, and the Germans were foolish to think that the Mexicans could offer them any substantive help. Still, have you never read about Pancho Villa's murderous raids north at the same time into American territory, and Pershing's punitive expedition against him? Think that happened just because Villa goofed?
And I don't agree that just because Mexico might be glad right now to get rid of population it can't support and to get their remittances from the US to float its economy, it doesn't have restoration of the American Southwest to its national territory as its long-term goal. Too many prominent Mexicans have posited this as a national goal over the last 150 years, and even more importantly, the average Mexican views the Southwest as legitimately Mexican territory. Postulating that the Mexicans "need" us economically and would thus never do anything to spoil US-Mexican relations takes no account of the broader history of US-Mexican relations or the national feelings of the Mexican people (often considerably stronger than "one-worlder" American elite opinion), and it is self-deluded, short-sighted and dangerous to construct an immigration and national security policy around it.
If there ever is a nuclear strike on the US, Mexico will probably have a really severe problem with illegal immigration from the US.
You think Mexico will have a problem absorbing US territory in the Southwest if we're prostrate from a nuclear attack??? Say good-night.
Yes, precisely BECAUSE we would be prostrate from a nuclear attack. Critical infrastructure in the region would be destroyed, as would the means of repairing same; the survivors would head for any place better than where they were.
As bad as Mexico is, it would be better than a Southwest where major urban areas and the transportation, electrical, and water networks had been destroyed by a large-scale nuclear strike. Many survivors would thus head south (and many of these survivors would actually be illegal immigrants coming back home).
In turn, Mexico would be forced to try and stem this flow, because that many people coming south all at once would overload THEIR infrastructure.