What are the reasons the Aztec and Incas were declined?

The decline of the Aztec and Inca civilizations was primarily due to Spanish conquest, which was facilitated by superior weaponry and military tactics, as well as the spread of diseases like smallpox that devastated their populations. Internal strife and social unrest also weakened these empires, making them more vulnerable to external threats. Additionally, the Spanish exploited existing rivalries, forming alliances with discontented tribes, which further contributed to the downfall of these powerful civilizations.