When Hernán Cortés landed on the Mexican coast in 1519 with fewer than 600 Spanish soldiers, he embarked on what seemed an absurdly ambitious mission: to conquer the Aztec Empire, a civilization that ruled millions across central Mexico. By August 1521, Tenochtitlan had fallen, the empire was destroyed, and Cortés had achieved one of the most dramatic military feats in human history. The victory was not won by Spanish steel alone. It resulted from a complex interplay of indigenous alliances, European technology, biological catastrophe, and Cortés’s own ruthless genius. This article breaks down exactly how Cortés conquered the Aztec Empire—examining the forces that made the impossible possible and the legacy that still shapes Mexico today.

The Aztec Empire on the Eve of Conquest

To understand the conquest, you must first understand what Cortés faced. The Aztec Empire—properly called the Triple Alliance of Tenochtitlan, Texcoco, and Tlacopan—was the dominant power in Mesoamerica. It extracted tribute from dozens of subject city-states, maintained a professional military, and ruled a capital city of perhaps 200,000 people, larger than any European city of the time.

Political Structure and Vulnerabilities

The empire was a tributary hegemony rather than a unified state. Conquered peoples kept their local rulers and customs but paid heavy tribute in goods, labor, and victims for human sacrifice. This system generated immense wealth for Tenochtitlan but also bred deep resentment. Many subject cities had been conquered within living memory and maintained strong identities. They were subjects, not citizens, waiting for any opportunity to throw off Aztec domination.

The tlatoani (emperor) held centralized authority, which created a single point of failure. When Cortés captured Moctezuma II, the entire political structure was temporarily paralyzed. This rigidity, combined with the resentments of tributary states, proved fatal.

Cultural and Military Limitations

Aztec warfare emphasized capturing prisoners for sacrifice over killing enemies in battle. Elite warriors sought to take captives alive, a practice that gave Spanish soldiers—who fought to kill—a tactical edge. Obsidian-edged swords and cotton armor were effective but no match for Toledo steel and plate armor. And while Aztec armies could number in the tens of thousands, they lacked the cavalry, firearms, and coordinated combined-arms tactics that the Spanish employed.

Cortés: The Man Behind the Conquest

Hernán Cortés was born in 1485 in Medellín, Extremadura, a poor region that produced many conquistadors. He studied law at the University of Salamanca, gaining the rhetorical and legal skills that later proved invaluable. Driven by ambition for wealth, glory, and the spread of Christianity, he sailed to the Caribbean in 1504. After participating in the conquest of Cuba, he was selected by Governor Diego Velázquez to lead an expedition to Mexico.

But Cortés was no mere obedient agent. When Velázquez tried to revoke the commission, Cortés sailed anyway, defying his superior and gambling that success would legitimize his insubordination. This act of bold, calculated risk-taking characterized his entire campaign.

The Expedition’s Composition

When Cortés left Cuba in February 1519, his force included 11 ships, about 600 soldiers, 16 horses, and a few small cannon. By any rational calculation, this was insufficient to conquer an empire of millions. But Cortés understood that military power alone would not win. He would need to turn the empire’s own subjects into his army.

Phase One: Alliances and the March Inland

The Tlaxcalan Alliance

After landing at Veracruz, Cortés moved inland. His first major test came in Tlaxcala, a fiercely independent confederation that had resisted Aztec conquest for decades. The Tlaxcalans initially fought the Spanish, but after several battles they recognized both the Spanish military advantage and the opportunity to finally defeat their Aztec enemies.

They made a strategic decision to ally with Cortés, providing tens of thousands of warriors. This alliance was the single most important factor in the conquest. Without Tlaxcalan forces, the Spanish could never have mustered the numbers needed to besiege Tenochtitlan or control conquered territories. The Tlaxcalans supplied food, intelligence, porters, and a secure base for retreat and resupply.

Cortés sealed the alliance through careful diplomacy, promising the Tlaxcalans liberation from Aztec tribute and favorable treatment under Spanish rule. Though these promises were later broken, the alliance held through the entire war.

The Cholula Massacre

In October 1519, Cortés received intelligence—possibly fed by his Tlaxcalan allies—that the city of Cholula was planning an ambush. He preemptively struck, ordering a massacre that killed several thousand unarmed nobles and civilians in the main plaza.

The massacre was calculated terror. Word spread throughout central Mexico that resistance invited annihilation while submission might allow survival. Many cities thereafter submitted peacefully, intimidated by Cholula’s fate. Cortés understood that showing mercy only after demonstrating extreme violence could achieve more than winning a dozen battles.

Entering Tenochtitlan

In November 1519, Moctezuma II made the fateful decision to welcome Cortés into Tenochtitlan. Why? Historians debate the reasons. Some cite Moctezuma’s religious beliefs—legends of the returning god Quetzalcoatl may have made him uncertain whether Cortés was divine. Others point to political calculation: Moctezuma wanted to size up the newcomers and contain them within the city where he could control them.

Whatever his reasoning, the decision was catastrophic. Within days, Cortés took Moctezuma hostage, ruling the empire through the puppet emperor. For months, Spanish soldiers were quartered in the palace, extracting gold and intelligence while the Aztec nobility fumed.

The Crisis: Noche Triste and Near-Disaster

Narváez Expedition and the Toxcatl Massacre

In May 1520, Cortés learned that Governor Velázquez had sent Pánfilo de Narváez to arrest him. Leaving Tenochtitlan with most of his men, Cortés marched to the coast, defeated Narváez in a daring night attack, and then recruited Narváez’s troops into his own force—turning a threat into reinforcements.

But in his absence, Pedro de Alvarado, left in command in Tenochtitlan, ordered the massacre of Aztec nobles during the Toxcatl festival. The massacre destroyed any remaining trust. When Cortés returned, he found the city in open rebellion. Moctezuma, sent to calm the crowd, was killed—either stoned by his own people or murdered by the Spanish. His brother Cuitláhuac became tlatoani and organized a fierce resistance.

The Night of Sorrows

On June 30, 1520, Spanish forces attempted to flee Tenochtitlan under cover of darkness. They were discovered while crossing the causeways, and Aztec warriors attacked from all sides—from the causeways, from canoes, and from rooftops. Loaded down with gold, many Spanish soldiers drowned or were captured and sacrificed. Perhaps half the Spanish force and thousands of Tlaxcalan allies died in what the Spanish called the Noche Triste (Night of Sorrows).

This was Cortés’s darkest hour. He wept under a tree (the “Tree of the Sad Night”) and later wrote that he had come closer to total defeat than at any other time. But rather than retreating to Cuba, he led the survivors to Tlaxcala, where his allies gave him shelter and time to rebuild.

Phase Two: The Devastation of Smallpox

Even as Cortés regrouped, disease was doing the work he could not. An African slave in Narváez’s expedition had carried smallpox, and the virus spread explosively through central Mexico during the summer and fall of 1520.

Smallpox killed an estimated 25 to 50 percent of the indigenous population in that first wave. It killed Cuitláhuac, the tlatoani who had expelled the Spanish, after only 80 days in power. His successor, Cuauhtémoc, inherited a city where famine and disease had already decimated the population.

The epidemic created a devastating asymmetry: Spanish soldiers and their Tlaxcalan allies, who had some immunity from childhood exposure, were largely spared, while the Aztecs died by the thousands. The invisible ally of disease helped tip the balance irreversibly.

Learn more about the role of smallpox in the conquest from National Geographic.

Phase Three: The Siege of Tenochtitlan

Cortés spent months in Tlaxcala rebuilding his force. He gathered more indigenous allies—not only Tlaxcalans but also Texcocans and others who saw the Spanish as the winning side. By spring 1521, his coalition numbered perhaps 100,000 to 200,000 warriors, the vast majority indigenous.

Cortés also solved the problem of Tenochtitlan’s island location. He ordered the construction of 13 brigantines—small sailing ships armed with cannon—to be assembled on Lake Texcoco. Timber was transported overland from the coast, and indigenous labor built the fleet. The brigantines gave the Spanish naval supremacy on the lake, cutting off canoe resupply and allowing them to bombard the city.

The siege began in May 1521. Spanish strategy was systematic: cut off food and water, destroy causeways to prevent escape, and advance neighborhood by neighborhood. Aztec defenders fought with desperate courage, contesting every house and canal. But they were starving, sick, and outnumbered.

The final stand took place in Tlatelolco, the northern district, on August 13, 1521. Cuauhtémoc was captured while attempting to flee by canoe. The city lay in ruins, with perhaps 100,000 dead from fighting, starvation, and disease. The Aztec Empire had ceased to exist.

Why the Conquest Succeeded: A Synthesis of Factors

1. Indigenous Allies

The Spanish did not conquer the Aztec Empire alone. Tens of thousands of Tlaxcalan, Texcocan, and other indigenous warriors provided the overwhelming majority of combat power. Without them, the expedition would have been impossible.

2. European Military Technology

Cavalry, steel swords and armor, crossbows, arquebuses, and cannon gave the Spanish a qualitative edge. But these advantages alone could not have beaten the empire’s numbers. They functioned as force multipliers that made indigenous allies more effective.

3. Smallpox and Disease

Epidemic disease killed perhaps half the indigenous population in the first wave. It removed key leaders, crippled the army, and demoralized survivors. This was the single most decisive factor, and it was entirely beyond human control.

4. Aztec Political Vulnerabilities

The tributary system created subject peoples eager to rebel. Centralized authority under the tlatoani created a single point of failure. Cortés exploited both with brilliant political maneuvering.

5. Cortés’s Leadership

Cortés combined audacity, ruthlessness, and strategic sophistication. He built and maintained alliances, employed terror effectively, captured enemy leaders, adapted to setbacks, and never lost sight of his objective. His leadership turned favorable conditions into total victory.

For a broader perspective, see Britannica’s entry on the conquest of Mexico.

Legacy and Consequences

Demographic Catastrophe

The population of central Mexico fell from an estimated 15–25 million in 1519 to about 1–2 million by 1600—a decline of over 90 percent. This was caused by disease, war, forced labor, and social collapse.

Cultural Destruction

Spanish authorities destroyed temples, codices, and religious objects. Indigenous religions were suppressed and replaced with Catholicism, though many traditions survived through syncretism. The loss of Aztec writings represents an irreplaceable loss of human knowledge.

The Birth of New Spain

Tenochtitlan was rebuilt as Mexico City, the capital of the Viceroyalty of New Spain. Colonial systems of governance, economy, and social hierarchy (including the caste system) were imposed, shaping Mexico for the next three centuries.

The Mestizo Society

Intermarriage and sexual relationships between Spanish men and indigenous women produced a mixed-race population that became the demographic majority. Modern Mexican identity is deeply rooted in this blending of indigenous and European heritage.

To explore the ongoing debate about Cortés’s legacy, read History.com’s analysis of his impact.

Conclusion

The conquest of the Aztec Empire was not a simple story of European superiority. It was a clash of civilizations in which Spanish military technology, indigenous alliances, epidemic disease, and Cortés’s own strategic brilliance combined in a perfect storm of destruction and transformation. The empire fell not because the Aztecs were weak, but because they faced multiple, simultaneous threats that no single leader or culture could have withstood.

The legacy of 1521 still shapes Mexico—its language, its religion, its racial and social dynamics, and its complex relationship with its indigenous past. Understanding how Cortés conquered the Aztec Empire means understanding the forces that created modern Latin America, and the profound, often painful, history that continues to echo across the centuries.