The Aztec Empire, Mesoamerica’s dominant power in the 15th and early 16th centuries controlled a capital city that was one of the largest in the world. Itzcoatl, named leader of the Aztec/Mexica people in 1427, negotiated what has become known as the Triple Alliance—a powerful political union of the city-states of Mexico-Tenochtitlán, Tetzcoco and Tlacopán. As that alliance strengthened between 1428 and 1430 it reinforced the leadership of the Aztecs, making them the dominant Nahua group in a landmass that covered central Mexico and extended as far as modern-day Guatemala.

And yet Tenochtitlán fell into decline after the siege and destruction of the city by the Spanish in 1521—less than two years after Hernándo Cortés and Spanish conquistadors first set foot in the Aztec capital on November 8, 1519. How did Cortés manage to overthrow the seat of the Aztec Empire?

When Spanish conquistadors arrived in the Aztec imperial city in 1519, Mexico-Tenochtitlán was led by Moctezuma II. The city had prospered and was estimated to host a population of between 200,000 and 300,000 residents.

At first, the conquistadors described Tenochtitlán as the greatest city they had ever seen. It was situated on a human-made island in the middle of Lake Texcoco. From its central location, Tenochtitlán served as a hub for Aztec trade and politics. It featured gardens, palaces, temples and raised roads with bridges that connected the city to the mainland.

Other city-states were forced to pay periodic tributes to Tenochtitlán’s public markets and to its religious center, the Templo Mayor or “Great Temple.” Religious tributes sometimes took the form of human sacrifices. While the Aztec’s monetary and religious demands empowered the empire, it also fostered resentment among surrounding city-states.

Hernándo Cortés formed part of Spain’s initial colonization efforts in the Americas. While stationed in Cuba, he convinced Cuban Governor Diego Velázquez to allow him to lead an expedition to Mexico, but Velázquez then canceled his mission. Eager to appropriate new land for the Spanish crown, convert Indigenous people to Christianity and plunder the region for gold and riches, Cortés organized his own rogue crew of 100 sailors, 11 ships, 508 soldiers and 16 horses. He set sail from Cuba on the morning of February 18, 1519, to begin an unauthorized expedition to Mesoamerica.

Arriving on the Yucatán coast, Cortés encountered Indigenous people who told him about other Europeans who had been shipwrecked and captured by local Mayans. Cortes freed Jerónimo de Aguilar, a Franciscan friar, from the Mayans and made Aguilar part of his crew. Aguilar turned out to be an invaluable asset to Cortes due to his ability to speak Chontal, the local Mayan language. With Aguilar at his side, Cortés and his conquistadors continued traveling the region, battling Indigenous groups along the way.

Cortés and his men then acquired another asset when an Aztec chief gifted them some 20 enslaved young Mayan women, including Malinalli, a Nahua woman from the Mexican Gulf Coast. Malinalli became baptized with the Christian name Marina and was later known as La Malinche. La Malinche spoke both the Aztec language of Náhuatl and Mayan Chontal and worked alongside the Spanish invaders, providing the conquistadors with the ability to communicate with any Indigenous groups they encountered.

With La Malinche and Aguilar in tow, the conquistadors made their way to the island city of Tenochtitlán where they were initially welcomed by Emperor Moctezuma II. When Cortés became concerned that Moctezuma's people would turn against his men, he placed Moctezuma under house arrest and Cortés attempted to rule through the detained Moctezuma.

Soon Cortés received word that the Cuban governor had sent a Spanish force to arrest Cortés for insubordination. Leaving his top lieutenant Pedro de Alvarado in charge of Tenochtitlán, Cortés took men to attack the Spanish forces at the coast. Cortes's men defeated the troops and took the surviving Spanish soldiers back with him as reinforcements to Tenochtitlán. In Cortés' absence, Alvarado had hundreds of Aztec nobles killed during a ceremonial feast, leading to further unrest among the Aztec people.

Tenochtitlán residents demanded the Spanish be removed from the city. When the detained Moctezuma could no longer control Tenochtitlán’s residents, the Spaniards either allowed him to die during a skirmish in 1520 or killed him—depending on varying accounts.

Driven from the capital, the Spanish later circled back with a small fleet of ships. Working in alliance with some 200,000 Indigenous warriors from city-states, particularly the Tlaxcala and Cempoala (groups who had resented the Aztec/Mexicas and wanted to see them vanquished), the Spanish conquistadors held Tenochtitlán under siege from May 22 through August 13, 1521—a total of 93 days.

With Tenochtitlán encircled, the conquistadors relied on their Indigenous allies for key logistical support and launched attacks from local Indigenous encampments. Meanwhile, another factor began to take its toll. Unbeknownst to the Spanish, some among their ranks had been infected with smallpox when they had departed Europe. Once these men arrived in the Americas, the virus began to spread—both among their indigenous allies and the Aztecs. (Some research has suggested that salmonella, not smallpox, had weakened the Aztecs.)

The first known case reportedly emerged in Cempoala—one of the city-states that had allied with the Spanish—when an enslaved African came down with the disease. The virus then spread. As the Spaniards and their allies later attacked Tenochtitlán, even when they lost battles, the smallpox virus infected the Aztecs. Aztec troops, members of the noble class, farmers and artisans all fell victim to the disease.

While many Spaniards had acquired immunity to the disease, the virus was new in the Americas and few Indigenous understood it. The bodies of smallpox victims piled up in the streets of Tenochtitlán and, with the city under siege, there were few available ways to dispose of the bodies.

Spaniards and their allies were taken in as prisoners (the Aztecs tended to hold captured prisoners for sacrifice to the gods, rather than kill them in battle) and traces of the virus were left on the clothes, hair and on dead bodies of those who had had the disease. As Tenochtitlán residents contracted smallpox they had no place to turn for help. Aztec priests and medicinal practitioners knew of no remedy and Tenochtitlán residents had little immunity.

The conquistadors arrived in Mesoamerica with steel swords, muskets, cannons, pikes, crossbows, dogs and horses. None of these assets had yet been used in battle in the Americas. The Aztecs fought the Spanish with wooden broadswords, clubs and spears tipped with obsidian blades. But their weapons proved ineffective against the conquistadors’ metal armor and shields.

When the Spanish arrived in the Americas they came from a war-oriented culture that had seen battle against other European nations for dominance and against North Africans for sovereignty. The conquistadors arrived in Mesoamerica with better guns and had been trained in tactical strategies. They deployed a cavalry that could chase down retreating warriors, dogs trained to track down and encircle enemies and horses capable of trampling adversaries.

Up against large armies of Spanish and Indigenous forces, surrounded and cut off from the mainland, and with a population succumbing to an unknown, devastating virus, the Aztec Empire was unable to fight off the invading Spanish conquistadors. The Aztecs, including members of the Aztec royal family—then were forced to adjust to life under Spanish rule.

"Cada Uno En Su Bolsa Llevar Lo Que Cien Indios No Llevarían: Mexica Resistance and the Shape of Currency in New Spain, 1542-1552.”  by Allison Caplan, American Journal of Numismatics (1989-), vol. 25, 2013, pp. 333–356. JSTOR.

“Jeronimo de Aguilar,” American Historical Association.

“Aztec Warfare Imperial Expansion and Political Control,” by Ross Hassig, University of Oklahoma Press, 1988, p. 244.

“Searching for the Secrets of Nature The Life and Works of Dr. Francisco Hernández,” by Dora B. Weiner, Stanford University Press, 2000, p. 86.

“Viruses, Plagues, and History Past, Present, and Future,” by Michael B. Oldstone, Oxford University Press, 2020, p. 46.

“So Why Were the Aztecs Conquered, and What Were the Wider Implications? Testing Military Superiority as a Cause of Europe's Pre-Industrial Colonial Conquests,” by George Raudzens. War in History, vol. 2, no. 1, 1995, pp. 87–104. JSTOR. Accessed May 18, 2021.