Final answer:
Cortés took Moctezuma II hostage for control over Tenochtitlán, leading to Moctezuma's death during a conflict. Cortés fled but later returned, captured the city, and began building Mexico City.
Step-by-step explanation:
When Hernán Cortés first entered the city of Tenochtitlán in 1519, he was initially welcomed by the Aztec ruler, Moctezuma II. However, the relationship quickly soured as Cortés took advantage of the situation to take Moctezuma hostage in an attempt to gain control over the city and its resources. Conflict ensued, and during the struggle, Moctezuma was killed, either by the Spanish or potentially by his own people, as they might have felt that he betrayed them. Subsequently, Cortés fled the city but returned with reinforcements to lay siege to Tenochtitlán, eventually conquering the Aztecs and razed the city to establish what is now Mexico City.