The downfall of the Aztec Empire was precipitated by a confluence of factors, the most critical being the Spanish conquest led by Hernán Cortés in 1519. The Spanish, equipped with superior weaponry and known for their alliances with discontented Indigenous groups, defeated the powerful Aztec forces. Additionally, the introduction of diseases such as smallpox devastated the Aztec population, significantly weakening their societal structure. Internal dissent, along with the spiritual interpretations of the arrival of the Spanish as an omen, further destabilized the empire. The fall of Tenochtitlán in 1521 marked the end of the Aztec Empire and the beginning of Spanish colonization.