Final answer:
The decline of the Aztec civilization was due to a combination of climate change, warfare, and particularly the Spanish invasion led by Cortés in 1521. The spread of diseases like smallpox significantly reduced the indigenous population, and the imposition of Christianity and destruction of cultural sites by the Spanish further contributed to the Aztec collapse.
Step-by-step explanation:
Key Factors Leading to the Decline of the Aztecs
The decline of the Aztec civilization can be attributed to several key developments, notably climate change, warfare, and the invasion led by Spanish conquistador Hernán Cortés in 1521. The Aztecs, with their sophisticated society and numerical strength, were weakened significantly due to these factors. The Spanish invasion, especially, brought with it alliances with local tribes, such as the Totonacs, who saw the Spanish as liberators from Aztec oppression. These alliances, coupled with the Spanish's superior military tactics and weaponry, gravely undermined the Aztec defense.
The introduction of deadly diseases such as smallpox and salmonella by the Spanish to the Aztec population had catastrophic effects. The diseases spread rapidly through the densely populated cities and along the heavily-traveled roads of Central Mexico, annihilating large numbers of the indigenous population, with estimates suggesting a decline from 25 million to 2 million over the span of approximately 60 years.
The fall of Tenochtitlán after a three-month siege marked the end of the Aztec Empire. The Spanish then destroyed Aztec temples and imposed Christianity, eroding the Aztec cultural hegemony. The consequent dismantling of the Aztec knowledge base meant the loss of their advanced understanding in areas such as astronomy and engineering technologies.