The fall of the Aztec Empire was caused by a combination of internal strife and external forces, particularly the arrival of Spanish conquistadors led by Hernán Cortés. The Spanish took advantage of existing resentments among subjugated tribes, forming alliances that increased their military strength. Cortés's forces captured the Aztec capital, Tenochtitlan, in 1521, helped by advanced weaponry and the spread of diseases like smallpox, which devastated the indigenous population. Moreover, the Aztecs faced political instability and civil wars, weakening their capacity to resist the Spanish invasion. Ultimately, these factors culminated in the rapid decline of the empire.