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Mexican President Herrera prior to the Mexican War was willing to negotiate a sale of territory with the United States. The US would get all of Texas's claims minus the Nueces Strip and would purchase California and New Mexico north of the 37th parallel (the Mexicans being aware of where American settlers were flooding into California and of the 36-30 line of slavery in the US).
Let's say that this exchange happens and land-hungry Southern Politicians, realizing they've just been boxed in, push hard for the annexation of the Republic of Yucatan. The bill for the annexation of Yucatan had historically been passed in the House of Representatives and was blocked in the Senate because the Senate thought that being dragged into the Caste War would be far too much of a headache with the country already at war with Mexico.
The Mexican War historically was already considered an incredibly amoral thing by many including future presidents like Abraham Lincoln and Ulysses S Grant. A drawn out protracted struggle in the Yucatan (a conflict that historically went on until the 1920s) would likely be much more morally outrageous. Perhaps it'd arouse antislavery sentiment a few years sooner?
Militarily many of the generals who cut their teeth in Mexico would possibly be fighting in Yucatan instead.
What if the US avoided the Mexican War and spent the late 1840s (and onwards) fighting a Yucatan Caste War?