
For once, alternate history goes mainstream and isn't about the Civil War or WWII. This time, the Nazis don't win, the Confederates don't win. Mexico wins:


(Courtesy of poor targeted advertising)
Sure, it's an ad for vodka, but it's also got a map, and I love maps. Now I know next to nothing about the Mexican-American war, where the US gained the land that makes up its current boundaries. I can't point to some specific thing that could have been changed in 1848 to tip the balance in favor of Mexico.
There's only one thing that I do know. Within the US, the Democratic party of the time supported the jingoistic war with Mexico and the opposition Whigs, generally more anti-slave, were split over support. When the US won, anti-war Whigs fell out of favor. Among these were Abraham Lincoln and many members of his cabinet. If the US had outright lost, the Democratic party would have taken a huge hit in popularity at the same time the Whigs were in deepest crisis. Instead of breaking up as they did - into pro-slave elements that largely joined the democrats and anti-slave coalitions that swept up other constituencies into the anti-slave Republican party - the Whigs might have been able to go though an election without a real opposition. In power for at least four years, they might have been able to coalesce into a united party again. Thus, the slave issue would not be sorted out by political parties.
In addition, a western politician like Lincoln who opposed the war vocally and from the start would have been especially viable as a candidate. Combine that with Lincoln's double-talk on slavery and you might have a powerful candidate. So maybe Lincoln would have been president anyway; the Democrats certainly would not have held the office for a couple terms.
Thank you, Absolut, for caring about alternate history in a creepily ethno-nationalist way! I'm sure your ad will go over well with all dozen serious supporters of an 'American Reconquista.'