The Aztec Empire was destroyed by a combination of Spanish conquistadors, their indigenous allies, and epidemic disease. Hernán Cortés led a small Spanish force into Mexico in 1519, but it was the tens of thousands of native warriors from rival city-states, along with devastating outbreaks of smallpox and other epidemics, that ultimately brought down one of the most powerful civilizations in the Americas. The capital city of Tenochtitlan fell on August 13, 1521, after a brutal 75-day siege.
Cortés and the Spanish Expedition
Hernán Cortés arrived on the coast of Mexico in 1519 with roughly 500 soldiers, a handful of cannons, crossbows, steel swords, and about 16 horses. The Aztecs had never encountered horses or gunpowder weapons. Their primary close-combat weapon was the macuahuitl, a wooden club lined with razor-sharp obsidian blades. It was a fearsome tool, but obsidian shattered against steel armor, and Spanish cavalry could break through formations of foot soldiers with devastating effect. Horses proved to be the single most decisive military advantage the Spanish held.
Still, 500 men could not topple an empire of millions. What Cortés had in equal measure to weaponry was political cunning. He quickly recognized that the Aztec Empire ruled through fear and tribute, and that many subject peoples resented their overlords deeply.
The Alliance That Made It Possible
Cortés’s most important weapon was diplomacy. As he marched inland, he forged alliances with indigenous groups who were hostile to the Aztecs, most crucially the Tlaxcalans. Tlaxcala was a powerful independent state that had resisted Aztec domination for decades. The Tlaxcalans eventually committed thousands of warriors to fight alongside the Spanish, and their contribution was enormous. By the final siege of Tenochtitlan, indigenous allies vastly outnumbered the Spanish soldiers in the coalition army.
Cortés also peeled away former Aztec allies, including the city of Tetzcoco, which had been one of the three partners in the Aztec Triple Alliance itself. This kind of political fracturing hollowed out the empire from within before the final battle ever started.
A key figure in making these alliances work was Malintzin, often called La Malinche. She was an indigenous woman who spoke both Nahuatl (the Aztec language) and Mayan. Working initially alongside a Spanish castaway who spoke Mayan and Spanish, she served as Cortés’s interpreter and diplomatic advisor. She negotiated with hostile tribes the Spanish had to pass through, uncovered at least two conspiracies that would have killed the expedition, and facilitated the critical meetings between Cortés and the Aztec ruler Moctezuma. Without her linguistic skills and political intelligence, the alliances that made the conquest possible may never have formed.
Smallpox and the First Wave of Death
Disease played a role that is hard to overstate. In 1520, a member of a rival Spanish expedition arrived on the Mexican coast carrying smallpox. The virus spread from his household in the coastal town of Cempoala to the surrounding population, and because indigenous peoples had no prior exposure and no immunity, it tore through communities at catastrophic speed. Between 5 million and 8 million people died in this first epidemic alone.
The timing was critical. In mid-1520, Cortés and his forces had just been driven out of Tenochtitlan in a bloody retreat. The Spanish were regrouping and desperate. The smallpox epidemic raged through the capital for about 60 days, killing huge numbers of Aztec warriors and leaders, including Moctezuma’s successor, Cuitláhuac. Those 60 days gave Cortés the breathing room he needed to rebuild his forces, construct a small fleet of boats, and launch his final assault.
The Siege of Tenochtitlan
Tenochtitlan was built on an island in Lake Texcoco, connected to the mainland by causeways. It was one of the largest cities in the world at the time. To take it, Cortés had boats built and launched on the lake to cut off food and fresh water to the city. The coalition of Spanish soldiers and indigenous warriors then fought their way in along the causeways over 75 days of grinding, street-by-street combat. On August 13, 1521, the last Aztec emperor, Cuauhtémoc, was captured and the city surrendered.
By that point, Tenochtitlan was in ruins. Its population had been decimated by fighting, starvation from the blockade, and the lingering effects of disease. The city that had awed the Spanish when they first saw it was largely destroyed.
The Epidemics That Followed
The fall of Tenochtitlan ended the Aztec Empire as a political entity, but the worst of the population collapse was still to come. A mysterious epidemic called cocoliztli (Nahuatl for “pest”) swept through the Mexican highlands in 1545, killing an estimated 5 million to 15 million people, or up to 80% of the remaining native population. A second cocoliztli outbreak in 1576 killed another 2 to 2.5 million, roughly half of whoever was left.
For a long time, historians assumed these later epidemics were also European diseases like smallpox, measles, or typhus. But more recent analysis suggests they may have been hemorrhagic fevers caused by an indigenous virus, possibly carried by rodents and triggered by drought conditions. The exact pathogen has never been conclusively identified, which makes these epidemics one of the more haunting mysteries in the history of the Americas.
The cumulative population loss was staggering. Estimates of central Mexico’s pre-conquest population range widely, from a few million to as high as 25 million. The most dramatic estimates, from historical demographers Sherburne Cook and Woodrow Borah, put the native population of central Mexico at 25.2 million in 1519, collapsing to 6.3 million by 1545, 2.5 million by 1570, and bottoming out at 1.2 million by 1620. Even more conservative scholars estimate losses of at least 50% across the colony, with some regions losing 90% or more of their population within a century.
The Aztec People Survived
The empire was destroyed, but the people were not erased. The Nahua, the ethnic group that built the Aztec Empire, are still one of the largest indigenous populations in Mexico. The 2000 Mexican census recorded nearly 1.4 million speakers of Nahuatl, the Aztec language, with over 2 million people living in households where it is spoken. More than 7,700 towns across Mexico have significant Nahuatl-speaking populations.
Nahua communities exist today across a wide geographic range, from the Sierra Madre Occidental to Guerrero, the central highlands, Puebla, and the Gulf Coast. The Spanish colonial system, for all its brutality, also created extensive written records in Nahuatl: wills, deeds, court records, and testimonies that continued for centuries after the conquest. These documents have given scholars an unusually detailed picture of how Nahua culture adapted and persisted through colonialism and into the modern era. For modern Nahua people, the word “Mexicano” carries a double meaning: citizen of modern Mexico, and proud descendant of the Aztecs.