"The Aztec Empire, or the Triple Alliance ... began as an alliance of three Nahua 'altepetl' city-states: Mexico-Tenochtitlan, Mexico-Texcoco, and Mexico-Tlacopan," according to Wikipedia. "These three city-states ruled the area in and around the Valley of Mexico from 1428 until they were defeated by the combined forces of the Spanish conquistadores and their native allies under Hernán Cortés in 1521."
The Aztec Empire is a classic example of what happens to regimes held together by force and oppression. These combinations are inherently unstable and don't last very long. In the case of the Aztecs, their empire only lasted around 93 years, but it's interesting to note that they still managed to hold together for about 24 years longer than the Soviet Union (1922 - 1991), which only survived for 69 years. That's a blink of the eye in historical terms.
Totalitarian states rise to power through military conquest and hang on to their rule by terrorizing their subject populations, but their strength is deceptive. In reality, empires like this are extremely fragile and vulnerable to overthrow by all the enemies they create. If their rule doesn't provide distinct advantages to the people they conquer (like the Roman empire), they're doomed. When the end came for the Aztecs, it came very suddenly. The Spaniards had no trouble assembling a "coalition of the willing" and destroyed the Aztec empire in less than two years.
Cortez was a hardcore soldier and adventurer. When he landed in Mexico with his small force and decided to conquer the Aztecs, he burned his ships so that retreat was impossible. After subduing some token native resistance, he made alliances with local tribes hostile to the bloodthirsty sun-worshiping Aztecs, advanced on the Aztec capital of Tenochtitlan and met Moctezuma II, the Aztec ruler. Forced to retreat by a mass rebellion, Cortez regrouped and captured the city after a siege. I'm just skimming over the events here, but that's essentially what happened. The Aztecs didn't have much of a chance against Cortez and his allies and they were also hindered by their religion:
"Unbeknownst to Cortés, his arrival coincided with an important Aztec prophecy. The Aztec god Quetzalcoatl, whom they credited with the creation of humans among other notable feats, was set to return to Earth. Thinking that Cortés could be Quetzalcoatl, Montezuma greeted the party with great honor." Source: Live Science.
Cortez was what the great Oswald Spengler would call a "culture-bearing" member of western civilization, which is driven by the Faustian impulse to explore, conquer and expand without limit. At the same time, his defeat of the Aztecs was a matter of private enterprise. According to Spengler, "the most appalling feature of the tragedy [the fall of the Aztec empire] was that it was not in the least a necessity of the Western Culture that it should happen. It was a private affair of adventurers, and at the time no one in Germany, France, or England had any idea of what was taking place. This instance shows, as no other shows, that the history of humanity has no meaning whatever and that deep significances reside only in the life-courses of the separate Cultures." -- The Decline of the West, Volume II: Perspectives of World History, p.44.