The Spanish conquest of the Aztec Empire (begun February 1519) was one of the most significant events in the Spanish colonization of the Americas.

The conquest must be understood within the context of Spanish patterns on the Iberian Peninsula during the Reconquista by Christians, defeating the Muslims, who had ruled the peninsula since 711. These patterns extended to the Caribbean following Christopher Columbus' establishment of permanent European settlement in the Caribbean. The Spanish authorized expeditions or entradas for the discovery, conquest, and colonization of new territory, using existing Spanish settlements as a base. Many of those on the Cortés expedition of 1519 had never seen combat before. In fact, Cortés had never commanded men in battle before. However, there was a whole generation of Spaniards who participated in expeditions in the Caribbean and Tierra Firme (Central America), learning strategy and tactics of successful enterprises. The Spanish conquest of Mexico had antecedents with established practices.[2]

The Spanish campaign began in February 1519, and was declared victorious on August 13, 1521, when a coalition army of Spanish forces and native Tlaxcalan warriors led by Hernán Cortés and Xicotencatl the Younger captured the emperor Cuauhtemoc and Tenochtitlan, the capital of the Aztec Empire.

During the campaign, Cortés was given support from a number of tributaries and rivals of the Aztecs, including the Totonacs, and the Tlaxcaltecas, Texcocans, and other city-states particularly bordering Lake Texcoco. In their advance, the allies were tricked and ambushed several times by the people they encountered. After eight months of battles and negotiations, which overcame the diplomatic resistance of the Aztec Emperor Moctezuma II to his visit, Cortés arrived in Tenochtitlan on November 8, 1519, where he took up residence, welcomed by Moctezuma. When news reached Cortés of the death of several of his men during the Aztec attack on the Totonacs in Veracruz, he took the opportunity to take Moctezuma captive, Moctezuma allowed himself to be captured as a diplomatic gesture. Capturing the cacique or indigenous ruler was standard operating procedure for Spaniards in their expansion in the Caribbean, so capturing Moctezuma had considerable precedent, which might well have included those in Spain during the Christian reconquest of territory held by Muslims.[3]

When Cortés left Tenochtitlan to return to the coast and deal with the expedition of Pánfilo de Narváez, Pedro de Alvarado was left in charge. Alvarado allowed a significant Aztec feast to be celebrated in Tenochtitlan and on the pattern of the earlier massacre in Cholula, closed off the square and massacred the celebrating Aztec noblemen. The biography of Cortés by Francisco López de Gómara contains a description of the massacre.[4] The Alvarado massacre at the Main Temple of Tenochtitlan precipitated rebellion by the population of the city. When the captured emperor Moctezuma II, now seen as a mere puppet of the invading Spaniards, attempted to calm the outraged populace, he was killed by a projectile.[5] Cortés, who by then had returned to Tenochtitlan and his men fled the capital city during the Noche Triste in June, 1520. The Spanish, Tlaxcalans and reinforcements returned a year later on August 13, 1521 to a civilization that had been wiped out by famine and smallpox. This made it easier to conquer the remaining Aztecs.

The fall of the Aztec Empire was the key event in the formation of the Spanish overseas empire, with New Spain, which later became Mexico, a major component.