The Aztec Empire was more a set of different peoples than a homogeneous and peaceful culture. The elite in Tenochtitlán oppressed city-states around it with heavy taxes and kept its dominance through a strong military.
Explanation:
This created many enemies of the Aztec Empire and so it happened that when the Spanish arrived these peoples joined forces with them to destroy Tenochtitlán.
It's important to remember that Aztec Empire it's not how they called themselves in the 1500s. "Aztec" is a term that began to be utilized only in the 1800s by European scholars. In the 1500s the people of Tenochtitlán called themselves Mexicas.
