Instead of being captured by Texian forces on 22 April 1836 in a marsh wearing a dragoon private's uniform, what if Santa Anna died at the Battle of San Jacinto just a day prior? This is just my personal take on the scenario; I encourage everyone to join in and post their thoughts in the comments below.
Alright, here we go:
On the 21st of April, 1836, the Battle of San Jacinto occurs. The results are still the same as in OTL with the Texan army routing the Mexicans; however, after the battle Texan troops find the dead body of Santa Anna, who was struck down by a stray bullet as he attempted to flee the battle. The news of Santa Anna’s death spreads like wildfire on both sides; upon reaching Mexico City, the Mexican Congress installs Anastasio Bustamante as president and orders General Vicente Filisola to continue on and crush the uprising in Tejas. Fighting continues throughout the rest of 1836 and into 1837, but eventually, Texas wins its independence from Mexico. An alternate version of the Treaties of Velasco is signed but this time, Mexico fully recognizes Texan independence and its claims. Texan annexation into the United States still occurs in 1845 as in OTL and isn’t responsible for souring US-Mexican relations.
Realizing at how fragile Mexico really was, Mexico City tries to reassert its authority in an attempt to prevent further secessionist movements from occurring. The Pastry War of 1838-39 occurs just like it did in OTL, but without Santa Anna to come out of retirement and lead Mexican forces against the French, Mexico’s military suffers a humiliating defeat.
By the time the 1840s roll around, Mexico is politically toothless, leaving her vulnerable to turmoil from within. As in OTL, the Mexican states of Coahuila, Nuevo León, and Tamaulipas break off to form the Republic of the Rio Grande (with claims in Durango, Chihuahua, and western New Mexico), the Yucatán peninsula breaks off to form the second Republic of Yucatán, and the Mexican state of Tabasco rebels and declares independence. However, without Santa Anna in charge and with the Mexican military in worse conditions, the uprisings in Rio Grande and Tabasco aren’t quickly crushed; instead, the two continue to exist as sovereign nations, although Mexico refuses to recognize either of them. Additionally, without Santa Anna, the Soconusco region in Chiapas is never occupied and remains a part of Guatemala to the present day.
After the annexation of Texas into the Union as the 28th state in 1845, President James K. Polk sends John Slidell to Mexico City, offering a cancellation of all Mexican debts to the U.S. and $40 million in exchange for Alta California and the remainders of Santa Fe de Nuevo Mexico. While the deal is initially met with strong resistance, Mexico soon agrees. Now in possession of a large amount of money and with additional troops from the north, Mexico quickly crushes the Republic of the Rio Grande and the independent state of Tabasco in late 1846. Debts to other nations, including debts to France from the Pastry War, are paid off. However, even with these positive events, Mexico’s political landscape remains unstable, with many fearing that a civil war was inevitable.
Additionally, the Republic of Yucatán still experiences the 1847 Caste War and still requests military aid to crush the native Maya population in exchange for annexation. This time however, since there is no Mexican-American War, the United States comes to the aid of Yucatán. Once the native Maya are crushed, the Yucatán peninsula is annexed and becomes a territory of the United States, and eventually a full-fledged US state in the 1850s.
Thanks for reading, I hope you all enjoyed this scenario!