I know it's cliche, but just about everything will change.
First and foremost in my mind is that the Civil War is likely averted, as the Missouri Compromise line can be easily extended to the Pacific with minimal fuss. Now, IOTL, both the Abolitionists and the Planters expected that slavery would fail to take root in Mexico but I'm not so sure.
Actually, I think northerners will be
particularly insistent on the Wilmot Proviso in the unlikely event All Mexico goes through. Many antislavery northerners had denounced the War as a slaveholders' conspiracy, and would hate the idea of an extension that could result in slavery going into not only California and New Mexico but potentially in some states south of the Rio Grande--at least the ones just to the south of it. And indeed the whole issue of slavery is one reason why I think All Mexico is so very unlikely. To quote (with a few minor changes) an old soc.history.what-if post of mine:
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From a reading of Frederick Merk's *Manifest Destiny and Mission in American History* (which has the best discussion I know of the movement for the acquisition of "all Mexico") I am convinced that the All Mexico movement was a phenomenon of the Northeastern penny press, and never had any real chance.
There were a number of reasons for this. Whigs, north and south, were vehemently opposed to the idea, and they had a majority in the House of Representatives. Besides, financing an occupation of Mexico would be expensive, and the Democrats were proud of having lowered rates with the Walker Tariff. Many of them were worried that a continued occupation of Mexico would force a return to high tariff rates (which might be attractive to Democrats from protectionist Pennsylvania but not most others).
The most important obstacle was racism and the slavery issue. On the one hand, antislavery Northerners denounced the Mexican War and any proposals for annexing Mexican territory as a slaveholders' conspiracy; yet on the other hand, some Southerners (the Whigs and Calhoun) opposed the war entirely, and few Southerners supported the acquisition of all Mexico. (The only Southern Democratic newspaper that shared the Northeastern penny press' enthusiasm for All Mexico was at the very northeastern edge of the South--Baltimore.) Both Calhoun and the Southern Whigs harped on the argument that the Mexicans were a "colored" people, who opposed slavery and would weaken it within the Union. And whatever their disagreements with Calhoun over the war itself, most Southern Democrats agreed with Calhoun when he said:
"I know further, Sir, that we have never dreamt of incorporating into our Union any but the Caucasian race--the free white race. To incorporate Mexico would be the very first instance of the kind, of incorporating an Indian race; for more than half of the Mexicans are Indians, and the other is composed chiefly of mixed tribes. I protest against such a Union as that!...Are you, any of you, willing that your States should be governed by these twenty-odd Mexican states...a mixed blood equally ignorant and unfit for liberty, not as good as the Cherokees or Choctaws?"
Calhoun also harped on the theme that administering Mexico would require precisely the kind of centralized national government the South feared (at least unless it was sure of controlling it!).
Note also the comments of Waddy Thompson, a South Carolina Whig who had spent some time as a diplomat in Mexico: "A friend said to me today that we will not take the people, but the land. Precisely the reverse will be the case; we shall take the people, but no land. It is not the country of a savage people whose lands are held in common, but a country in which grants have been made for three hundred and twenty-five years, many of them two and three hundred miles square...it is all private property, and we shall get no public domain which will pay the cost of surveying it. I speak of the country beyond the Rio Grande. We shall get no land, but we shall add a large population, alien to us in feeling, education, race, and religion..."
It might be thought that if proslavery Southerners opposed All Mexico as a menace to slavery, antislavery Northerners should have supported it for the same reason. However, the closest thing I have been able to find to this is the proposal of the antislavery *National Era* that the United States should unilaterally declare peace and should *invite* nineteen Mexican states (the ones with sufficient population) to enter the Union as states. That newspaper was convinced that doing this would fatally undermine the Slave Power. The people of these new states would all see to it that their states would remain non-slaveholding, and they were at least as fit for self-government as the hordes of immigrants now pouring into the US from Europe...But in the first place, the *National Era* emphasized that the entrance into the Union had to be voluntary; second, despite this qualification, the idea was denounced by other antislavery forces as "pandering" to the robber spirit of conquest; and third, as one might expect, it was unanimously denounced by Southerners. In any event, there was little chance of the Mexicans agreeing to this. It is true that some of the radical "Puros" so despaired of secularizing and reforming Mexico internally, they were prepared to get reform from without--by joining the United States. But even among the Puros, it's doubtful this was a widespread sentiment--certainly their leader Gomez Farias didn't feel that way.
One gets the impression that what most Americans wanted was as much Mexican territory as possible with as few Mexicans as possible. What convinces me of the superficiality of the sentiment for "all Mexico" is that even the expansionists actually seemed relieved at Trist's treaty, despite its insubordinate origins. Thomas Ritchie of the *Washington Union* spoke for many when he expressed happiness that the land taken from Mexico was encumbered by only 100,000 Mexicans.
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To that post, I would add just a few things:
(1) The support of the Northeastern "penny press" for All Mexico is understandable when you consider that they represented a polyglot region, and that their readers were largely immigrants, including Catholics. The rest of the country would be unlikely to share their perspective that non-Anglo-Saxons (and Catholics at that) could make good US citizens...
(2) I do not deny that some southerners wanted more accessions and even hoped that slavery could spread there. But saying "In addition to what we got under Trist's treaty, I want Coahuila and Tamaulipas and Nuevo Leon" is very different from saying "I want all Mexico."
(3) On the subject of the likelihood that some of the northern Mexican states, if incorporated into the US, could support slavery: Noel Mauer (who has considerable knowledge of Mexico: see
https://business.gwu.edu/noel-maurer for his background) had an interesting blog post on this some years ago:
"We have an example of a populated area switching to American rule. New Mexico had a population about as large as Coahuila's and a little more than half of Nuevo León or Chiahuahua. It provides a perfectly valid template for how those territories would have developed under American rule; with one wrinkle that I'll get to later.
"We also know what American troops experienced during the occupation. Mexican politicians in the D.F. were horrified at the level of indifference, shading over in many cases -- not least Nuevo León -- outright collaboration.
"The wrinkle, which would make Coahuila and Nuevo León different from New Mexico, is that the elites in the northeastern states actively desired American annexation and the extension of slavery. We know this because they asked for it! Santiago Vidaurri wrote a letter to Richmond in 1861 volunteering Coahuila and Nuevo León to the Confederate cause. (Vidaurri annexed Coahuila to N.L. and installed himself as the governor of Tamaulipas.)
"These sympathies predated the Civil War. In fact, Vidaurri had been perfectly happy in 1855 to return escaped slaves to Texas. The agreement failed because the Texans wanted to send in their own people to recapture the escapees, not principled opposition; ironically, he made a whole bunch of antislavery proclamations in 1857, only to reverse them and start sending slaves home in 1858. It is hard to believe that Vidaurri or the elites that supported him would have opposed slavery, given their opportunism and their incessant complaints about labor shortages..."
http://noelmaurer.typepad.com/aab/2014/10/what-would-lesser-mexico-have-been-like.html
(4) In any event, with or without "All Mexico," extension of the Missouri Compromise line to the Pacific was a very pro-southern solution to the slavery-in-the-territories problem. Hardly anyone expected slavery to flourish north of that line, while protecting slavery south of that line could set a precedent for future acquisitions in Mexico (if not all of it were taken at once), Central America, Caribbean islands, and anything down to Tierra del Fuego... (No wonder that even southerners who believed in principle that the federal government had a duty to protect slavery in all territories were nevertheless willing to accept extension of the Missouri Compromise line as an acceptable "compromise"!)
(5) Don't equate slavery with cotton--many southerners hoped (and northerners feared) that slave labor could also be used for mining. That's another reason why a lot of people both north and south did not regard the slavery extension debate as a mere abstraction.
