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I have a series of questions related to an early annexation of Texas by the United States, with a POD around 1837-38, and the impact this would have on subsequent American history.

In 1836, the Republic of Texas declared its independence from Mexico, and the following year it sent an annexation proposal to the United States. From what I understand President Martin Van Buren rejected the idea for fear of upsetting the sectional balance between the north and south and possibly causing a war with Mexico. This delayed Texas's statehood until 1845, which did in fact result in the Mexican-American War, as well as US acquisition of most of the modern day American west.

My first and most obvious question is, what if Van Buren had not been an opponent of annexation, but instead approved the initiative and sent it to Congress for a vote? Would it have passed? I admit this would be somewhat out of character given that he was a New Englander and didn't have a big interest in acquiring land that would mainly benefit the south (unlike Andrew Jackson, who apparently supported Texas annexation from the get go). However, I've read that Van Buren had a poor showing in the south and west during the election of 1836, and being the consummate politician that he was, I could see him seizing on this to bolster Democratic support there for the upcoming midterm elections. At the very least, it might be a tempting way to stem the growth of the Whig Party following the Panic of 1837, although at the expense of sectional unity.

Assuming for the moment that Van Buren receives the 1837 offer favorably, though, and that the Democratic Congress subsequently passes a treaty of annexation, what happens with Mexico? Do we see a war similar to the 1846-48 conflict break out ten years early, or does the US take a more cautious approach and accept the Nueces River boundary? I can think of a lot of factors that might give the Americans pause about risking war by marching into the disputed area, not least Van Buren's own desire to avoid it (as opposed to Polk's staunch Manifest Destiny) and the tensions with Britain over the Caroline Affair in December, 1837. Not having Polk in office would be the biggest obstacle, I imagine; Van Buren was not elected on a platform of national expansion and did not claim for himself the kind of mandate that Polk did in 1845. (This, of course, is also a good reason why he might not support annexation in the first place, but I digress.)

My own inclination is therefore that a Van Buren annexation would be far more diplomatic and reserved, aiming to placate the Mexicans and settle for Texas itself rather than goading them into attacking so as to acquire more territory. How Mexico responds to American overtures could be a vastly different story, though. They still wouldn't be keen on letting one of their largest states get away without a fight, and the lack of ten full years of Texan independence might further embolden them, as they might see Texas as being unable to defend itself in the long run without help. Then again, if the annexation occurs around the time of the Pastry War, Mexico might decide it doesn't want to alienate the United States, with its Monroe Doctrine, and could be more inclined to accept a permanent settlement. This would have some interesting butterflies for later American history, particularly the chain of events leading to the Civil War.

There are a lot of factors I have left out here, like the portion of New Mexico that Texas claimed and the Alta California uprisings, but Mexican history is not my strong point (neither is American history for that matter) so I was hoping that people could give general observations on this before I looked into it any deeper.
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