redlightning
Banned
Make it possible for Mexico to be able to launch a war of reconquest of the American southwest in what became part of the Mexican Cessation.
The Soviet Union and the United States trade nukes, and Mexico only gets hit with collateral fallout.
Before 1900?
Best,
Time traveling nukes are at least as plausible as anything else that could weaken the US after 1848, to the point that Mexico could launch a successful reconquest.
(Yes, I forgot which forum this was.)
You're looking at external invasion, but the only good time for that would be during the American Civil War, and Mexico was busy being invaded themselves at the time. Butterfly away Napoleon III's wife's ambitions (possibly by avoiding the marriage) so that France isn't looking to conquer Mexico, provoke Britain into war with the USA, and then Mexico might be able to make a successful invasion.
It'd be temporary, though - if the Union wins out, they're going to want their territory back, and if the Confederacy wins out, they're going to want it as 'rightfully theirs', and either one is strong enough to beat up on Mexico (though the Confederacy would need to rebuild strength first).
Go further than the Civil War, and the USA can now fend off any invasions from Europe successfully, leaving no opportunity for Mexico.