Answer:
Smallpox, the Aztecs welcoming the Spanish and the Spanish taking Moctezuma prisoner.
Explanation:
The smallpox greatly contributed to the decline of 90% Native Americans after the Spanish arrived. After the Aztecs welcomed Cortes and his crew with lavish gifts, they eventually wanted more of their wealth, which led Cortes to take Moctezuma as a prisoner and join up with the enemies of the Aztecs to destroy Tenochtitlan.