Final answer:
Cortés made an alliance with the Tlaxcalans who had not been conquered by the Aztecs. This alliance was pivotal in his conquest of the Aztec Empire, leading to the fall of Tenochtitlán and the rise of Mexico City on its ruins.
Step-by-step explanation:
In his way to meet Moctezuma, Cortés made an alliance with Tlaxcalans, a state that had not been vanquished by the Aztecs. The Tlaxcalans, bearing a traditional enmity towards the Aztec Empire, saw an opportunity in the arrival of the Spanish and their advanced weaponry. This alliance proved crucial as Cortés was able to leverage the Tlaxcalans' forces alongside his own to challenge and eventually defeat the Aztecs.
Hernán Cortés, the Spanish conquistador, ultimately took Moctezuma II, the Aztec emperor, hostage. The native peoples' resentment towards the Aztec hegemony catalyzed their willingness to ally with the Spanish. The fall of Tenochtitlán in 1521 marked a significant turning point in the conquest of the Aztec Empire and the establishment of New Spain, with Mexico City erected upon the ruins of the grand Aztec capital.