The Spanish conquest of the Aztec Empire was not just one of the most significant events in the Spanish colonization of the Americas but also in world history. Although the conquest of central Mexico was not the conquest of all regions in what is modern Mexico, the conquest of the Aztecs is the most significant overall.
The conquest must be understood within the context of Spanish patterns on the Iberian Peninsula during the Reconquista by Christians, defeating the Muslims, and also patterns extended in 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. Spanish conquest of Mexico had antecedents with established practices.
The 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 offered 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 peoples 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 in his own palace and ruled through him for months. 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.
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. 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 outraged Aztecs, he was killed by a projectile. Cortés, who by then had returned to Tenochtitlan, and his men had to fight their way out of the capital city during the Noche Triste in June, 1520. However, the Spanish and Tlaxcalans would return with reinforcements and a siege that led to the fall of Tenochtitlan a year later on August 13, 1521.
The fall of the Aztec Empire was the key event in the formation of New Spain, which would later be known as Mexico.