The fall of the Aztecs, 1520-1521
The Capture of Moctezuma
When Cortés and his force entered Tenochtitlán in November 1519, they were welcomed as guests but immediately faced a dilemma: they were vastly outnumbered inside a city they could not control. Cortés took a bold decision. Within a week of arrival, he took Moctezuma II hostage, keeping the emperor under Spanish guard while forcing him to continue appearing as ruler. The strategy worked for several months — the Aztec nobility complied with Moctezuma’s instructions, and Cortés used him to demand further tribute and gold.
The situation became unstable in May 1520. While Cortés left Tenochtitlán to deal with a new Spanish expedition sent by Governor Velázquez to arrest him, his deputy Pedro de Alvarado ordered a massacre of Aztec nobles gathered for the festival of Toxcatl. The number killed is debated — Spanish accounts suggest several hundred; some indigenous accounts suggest thousands. The massacre destroyed whatever cooperation the Aztec leadership had extended and triggered a general uprising.
The Toxcatl massacre (May 1520), ordered by Pedro de Alvarado in Cortés’s absence, destroyed Spanish-Aztec cooperation. The subsequent death of Moctezuma (June 1520) — whose exact circumstances remain disputed — removed the last possibility of negotiating through the emperor.
La Noche Triste, June 1520
Moctezuma died in June 1520. Spanish accounts say he was stoned by his own people when he appeared on a balcony to calm the uprising; indigenous accounts suggest the Spanish killed him. Whoever was responsible, his death was catastrophic for the Spanish position. The Aztecs elected a new emperor, Cuitláhuac, who immediately organised a full military assault on the Spanish garrison.
Cortés returned to Tenochtitlán to find his men trapped. On the night of 30 June–1 July 1520, the Spanish and their Tlaxcalan allies attempted to escape the city by night across one of the causeways. The Aztecs discovered the retreat and attacked. The Spanish called this disaster La Noche Triste (“the sad night”). Cortés lost approximately half his Spanish soldiers, most of his horses, and virtually all his artillery and gold. Many men drowned in the lake weighed down by gold they had looted from the treasury.
One of the most decisive factors in the Aztec defeat was not Spanish military superiority but epidemic disease. Smallpox, introduced by a soldier in Velázquez’s expedition (which arrived in 1520), spread through Tenochtitlán and the surrounding population. The indigenous population had no previous exposure to smallpox and therefore no immune resistance — a condition historians call “virgin soil epidemic.” Emperor Cuitláhuac himself died of smallpox within eighty days of taking power. Estimates of Aztec casualties from the 1520 smallpox outbreak range widely, but it clearly devastated the city’s population and military capacity at a critical moment. The disease spread ahead of Spanish armies, weakening peoples before the Spanish even arrived — making it arguably the single most important cause of the conquest.
The Siege of Tenochtitlán, 1521
After La Noche Triste, Cortés retreated to Tlaxcala, where his allies regrouped and resupplied him. He spent the remainder of 1520 rebuilding his force, constructing thirteen armed brigantines (flat-bottomed sailing vessels) for lake warfare, and gathering a vast allied indigenous army. The new Aztec emperor, Cuauhtémoc, had replaced Cuitláhuac (who died of smallpox) and organised a determined defence.
The siege began in May 1521. Cortés’s brigantines took control of the lake, cutting the city’s supply lines. Aztec defenders fought street by street; Cortés systematically demolished buildings to prevent them being reused as strongpoints, gradually reducing the city to rubble. The population suffered from starvation, disease and fighting. On 13 August 1521, Cuauhtémoc attempted to escape by canoe and was captured. The resistance collapsed. The siege had lasted approximately seventy-five days.
Cortés defeated Tenochtitlán through a combination of naval blockade (thirteen brigantines on Lake Texcoco), systematic demolition of the city’s defences, and a vast allied indigenous army — primarily Tlaxcalan — that provided the majority of the fighting force.
The Foundation of Mexico City
The fall of Tenochtitlán on 13 August 1521 ended the Aztec Empire. Cortés ordered the ruins of the city cleared and a new Spanish colonial city built directly on top — an act of deliberate symbolic erasure. Mexico City was constructed on the same island site, reusing Aztec stone and, initially, Aztec labour. The main Spanish cathedral was built on the site of the Templo Mayor — replacing Aztec religious architecture with Christian. This physical overwriting of the Aztec city was a statement about the permanence of Spanish power.
Cuauhtémoc was captured on 13 August 1521. Cortés initially kept him alive as a useful political figure who could help govern the conquered population. Cuauhtémoc reportedly asked Cortés to kill him rather than live as a prisoner, a request the Spanish did not grant. He was tortured in 1521 to make him reveal the location of hidden Aztec treasure. In 1525, Cortés had him executed, claiming he had discovered a plot against the Spanish. In modern Mexico, Cuauhtémoc is commemorated as a national hero — a symbol of indigenous resistance.