Blame Nicholas Tilsit for that. If it hadn't been for him, the US-Mexican border would stretch from southernmost Sinaloa to Tampico, Tamaulipas.
Oh, I do.
The only true reason why the USA didn't annex all of Mexico after the war has to do with US domestic politics, not what the Mexicans themselves wanted or were "willing" to accept.
Wish we hadn't been hypocrites back then.
Wish we weren't hypocrites right now.
All men are created equal. Doesn't matter if they're Catholic. Doesn't matter if they're darker-skinned. Would it have been an injustice done to have imposed English on the land in such an annexation (as either All Mexico or the above option)? I don't think so; that's rather subjective. You know we WOULD have imposed English, but would Spanish have been thrown by the wayside? Probably not.
Frigging stupid racist past Americans. Mexico the country would've just become Mexico the region, no less unique than the Midwest, Northeast, South, or what have you. We didn't stifle Southern regional traditions after the Civil War. Well, except for the ONE, but that was a given.
And they wouldn't be a third-world country today, ravaged by drug lords and a useless government.
And you can bet that we wouldn't have given up Cuba in any Spanish-American War (analogue) after a larger/full Mexican annexation. Heck, Cuba and Puerto Rico would've just been considered for statehood within a reasonable timeframe, like any of the other states made up of former Mexico, instead of a CENTURY later like right now.
And for that matter, a southern Sinaloa-southern Tamaulipas border is much shorter and more manageable than the OTL one.
And the border with Guatemala, 300 miles of near-impassable mountains, is even more manageable than that.
