The Aztec Empire: Rise of the Triple Alliance and Spanish Conquest

A City on a Lake That Astonished the World

When Spanish soldiers under Hernán Cortés first saw Tenochtitlan in November 1519, many reportedly wondered if they were dreaming. The conquistador Bernal Díaz del Castillo, who was present, wrote that "some of our soldiers even asked whether the things we saw were not a dream." Tenochtitlan—built on an island in the middle of Lake Texcoco—had a population of approximately 200,000–300,000 people, making it larger than any city in Spain and among the five largest cities in the world at that time. It was connected to the mainland by broad causeways, serviced by canoe traffic, and featured massive stone pyramids, palaces, a sophisticated water supply system, and markets that one Spaniard compared favorably to those of Salamanca or Constantinople. Within two years of that first view, the city was a ruin and the empire that built it was destroyed.

The Founding of Tenochtitlan, 1325

The Mexica—the people the Spanish would call Aztecs—arrived in the Valley of Mexico as a relatively minor migrant group around the early 13th century. According to their own historical traditions, they had wandered for generations from a mythical northern homeland called Aztlan, guided by their patron deity Huitzilopochtli. The Mexica tradition records that their god instructed them to found their city where they saw an eagle eating a snake while perched on a cactus—an image that appears on the modern Mexican flag. They found this sign on a small island in Lake Texcoco in 1325 CE and built Tenochtitlan. The location, though marshy and unpromising, offered crucial defensive advantages: access was possible only via causeways that could be flooded or barricaded. No army could approach undetected.

The Triple Alliance and Imperial Expansion

The Mexica were initially subordinate to the dominant power in the region—the Tepanec city of Azcapotzalco. In 1428, the Mexica ruler Itzcoatl formed the Triple Alliance with the neighboring cities of Texcoco and Tlacopan (Tacuba), overthrew Azcapotzalco, and rapidly built an empire across central Mexico. The division of tribute and conquered territory among the three alliance partners was formalized: Tenochtitlan received 2/5 of all tribute, Texcoco received 2/5, and Tlacopan received 1/5. In practice, Tenochtitlan increasingly dominated the alliance—its population and military power far exceeded its partners by the mid-15th century.

Flowery Wars and the Sacrificial Tribute System

The Aztec Empire's religious-military system required a continuous supply of captives for ritual sacrifice at the great temples. The Mexica practiced what they called "flowery wars" (xochiyaoyotl)—ritualized conflicts with neighboring powers, particularly the Tlaxcalans, specifically designed to capture live warriors rather than kill them in battle. Captives taken in flowery wars were brought to Tenochtitlan, fed and cared for, and then sacrificed in large-scale ceremonies atop the Templo Mayor. The precise scale of Aztec human sacrifice is debated: 16th-century Spanish sources cite figures of 20,000 or more sacrifices per year, which modern scholars consider exaggerated; archaeological evidence confirms large-scale sacrifice but supports lower estimates. The dedication ceremony of the expanded Templo Mayor in 1487 under Emperor Ahuitzotl reportedly involved the sacrifice of thousands of captives over four days—one of the largest single sacrifice events recorded in Mesoamerican history.

Sacrifice was theology, not cruelty.

The Pochteca Merchant Class

Trade was the sinew of the Aztec Empire. The pochteca were a specialized hereditary merchant class with their own patron deity (Yacatecuhtli), their own judicial system, and their own residential quarters in Tenochtitlan. Pochteca merchants organized long-distance trading expeditions to distant regions—the Gulf Coast, the Pacific coast, and Mayan territories in the south—exchanging luxury goods: quetzal feathers, jade, turquoise, cacao, and jaguar pelts. The pochteca also functioned as intelligence agents for the Mexica state, scouting territories before military campaigns. The market at Tlatelolco—Tenochtitlan's twin city on the same island—was described by the Spanish as capable of accommodating 60,000 traders on major market days.

Chinampas: Engineering Fertility in a Lake

The agricultural foundation of Tenochtitlan's population density was the chinampa system—artificial islands built in the shallow lake bed by layering alternating strata of mud and vegetation, anchored by willow trees. Chinampas could be farmed continuously year-round, producing multiple harvests of maize, squash, beans, and amaranth without soil depletion, because lake sediment periodically refreshed nutrient levels. The chinampa zone surrounding Tenochtitlan covered approximately 9,000 hectares at the empire's peak. Surviving chinampa systems still operate in the Xochimilco district of Mexico City—a UNESCO World Heritage Site and one of the few remaining examples of this pre-Columbian agricultural technology.

Hernán Cortés and the Alliance Against the Aztecs

Cortés landed on the Mexican coast in February 1519 with approximately 600 soldiers, 16 horses, and 14 cannon. The disparity between this force and the Aztec military—which could field armies of 80,000 or more—makes the conquest comprehensible only through the politics of Aztec imperialism. The Aztec tributary system had created numerous subject peoples with compelling reasons to assist any credible alternative power. Cortés found his most important allies in the Tlaxcalans—who had resisted Aztec conquest and suffered repeatedly in flowery wars—and in other groups resentful of Mexica dominance. By the time Cortés marched on Tenochtitlan, his Spanish force was embedded in an indigenous coalition numbering in the tens of thousands.

Smallpox and La Noche Triste

Smallpox—to which no indigenous American had any prior immunity—arrived in Mexico ahead of the Spanish army, spread by an infected member of a 1520 relief expedition. The epidemic that swept through Tenochtitlan in late 1520 killed Emperor Cuitláhuac (who had organized the successful uprising against the Spanish) after only 80 days of rule. Mortality estimates suggest 25%–50% of Tenochtitlan's population died in the initial epidemic.

Before smallpox did its worst, the Spanish garrison in Tenochtitlan was driven out in the disastrous "Night of Sorrows" (La Noche Triste) of June 30, 1520. Mexica warriors attacked the retreating Spanish forces on a causeway; hundreds of Spanish soldiers and thousands of Tlaxcalan allies drowned in the lake, weighed down by gold they refused to abandon. Cortés reportedly wept under a tree afterward. He rebuilt his force, constructed brigantines to control the lake, and began the final siege of Tenochtitlan in May 1521. The city fell on August 13, 1521. The last Mexica emperor Cuauhtémoc was captured while attempting to flee by canoe. He was later tortured and executed. Tenochtitlan became Mexico City, capital of New Spain.