Disease. When the Spanish arrived, they brought with them smallpox. Smallpox spread among the indigenous people and crippled their ability to resist the Spanish. The disease devastated the Aztec people, greatly reducing their population and killing an estimated half of Tenochtitlan's inhabitants.
The rise of the Aztec empire really began in 1150 with the fall of the Toltec empire. ... Their empire spread through most of central Mexico. After a period of droughts and internal factional conflict, the city collapsed and was burned and looted, possibly by the Chichimeca (the “wild” tribes to the north).
