Today in 1836 was the Battle of the Alamo. Antonio López de Santa Anna had been a war hero, of sorts, in the War of Mexican Independence, although he tended to inflate his actual role in the war, and then a war hero again defending Mexico from various incursions. He leveraged this fully to catapult himself to the forefront of Mexican politics, in which he became a champion for federalist government and democratic elections. He was elected President in 1833, but in fact seems to have been completely bored by the work of governing; he just let his vice president, Valentín Gómez Farías, do it all. Gómez Farías, having a free hand, initiated sweeping liberal reforms, mostly aimed at breaking the power of the army and of the Catholic Church, the two dominant forces in Mexican politics. So sweeping were the reforms that they encountered stiff opposition, and then Santa Anna stepped in and overturned them. He dissolved the Mexican Congress and began centralizing power. Multiple states rebelled at the sudden loss of federal safeguards for state power. The best equipped of the states, Zacatecas, was utterly crushed by Santa Anna's army, and then it went northward to quell the rebellion in Cuahila y Tejas, a region heavily populated by immigrants from the United States.
It was a heavier project than Santa Anna bargained before, in part because, despite the fact that he liked to style himself 'the Napoleon of the West', he lacked Napoleon's sense of logistics. Zapatecas was in central Mexico, rich and populated; Texas was at the edge of the Mexican empire across some relatively desolate country. Supply lines stretched thin. Nights grew cold, increasing illness among the soldiers and overloading the limited medical resources. Santa Anna had a poor staff organization, and so it seems many of the problems never came to his attention; those that did, he often seemed to have dismissed because he thought that it was temporary inconvenience and that he could crack the Texians quickly.
In Texas, in a reasonably competent move, Santa Anna split his forces, sending one group along the coast under General José de Urrea and leading the other himself to San Antonio de Béxar. (The two represented the major fortresses already in operation to which the rebels had access.) Urrea pushed to Goliad, and his campaign was a resounding success: over and over again Urrea won battles overwhelming the Texian army. Santa Anna, in the meantime, found at San Antonio a Texian garrison, which had taken over after defeating the previous Mexican garrison. They were holed up in the Alamo mission complex. For thirteen days, Santa Anna's thousand or so soldiers laid siege to the couple hundred Alamo defenders under the command of William Travis. An intense battle was fought on March 6, and the mission walls breached.
While all of this was going on, representatives from all over Texas had met and declared independence on March 2; Travis had sent a letter to them asking for reinforcements, but it did not arrive until the Alamo had already fallen. And Santa Anna's army marched on.
The Alamo was an easy win -- a minor affair, as Santa Anna called it, despite the fierce defense. There was no way Santa Anna could have lost, short of reinforcements that could not possibly have arrived. And the Goliad Campaign was an extraordinary success, as well. But more was going on than just the military victories. Texas was now committed to full-scale fight, having declared independence. Santa Anna expected (and he was not alone) that general report of the Alamo victory would demoralize soldiers; the effect was more complicated. And at Goliad, Urrea had promised to treat prisoners fairly and well. Santa Anna had them executed on March 27 -- almost five hundred of them. These things were adding up to something that Santa Anna could not quite grasp. Military-wise, the Texans were massively on the run -- what is known as the Runaway Scrape, the massive evacuation and flight of citizens and soldiers from the advancing Mexican army. Sam Houston, in charge of the last remnants of the Texan army, was in constant retreat, to the immense demoralization of his soldiers and the anger of the interim Texas government. Texas was collapsing before the Mexican juggernaut.
Then Santa Anna camped his army in a poorly chosen location not far from Lynchburg Crossing, a marshy area with lots of tall grass, and on April 21, Houston came full strength against a completely unprepared and surprised Mexican Army, with the Texians shouting, "Remember Goliad! Remember the Alamo!". The Battle of San Jacinto lasted eighteen minutes. Six hundred Mexican soldiers died (compared to eleven Texians) and Santa Anna himself was captured.
A significant portion of the Mexican Army remained under Urrea, who could have fought and could have won. But after weeks of negotiation, Santa Anna signed the Treaties of Valasco, and the war ended. The Mexican Army withdrew south. But nobody expected to end there; Urrea was gathering a bigger army, and everyone in Texas expected that he would be back soon. But it never quite happened. Santa Anna had been deposed (and notably the Mexican press and government were almost as harsh in their judgment of his Goliad decision as the Texans were), and there were still rebellions going on elsewhere. Eighteen minutes of vengeance for the Goliad Massacre and the Alamo had established Texas as a free republic.
Santa Anna would eventually push his way back into power by fighting off a French invasion, and try to invade again; it was fought off, but it was this, perhaps more than anything else, that really pushed Texas into considering annexation by the United States. Santa Anna would later be deposed again. Then he got back into power again. It's one of the features of his career that really does make him a sort of Napoleon of the West.