The Aztec (Mexica) empire at the time of Spanish contact in 1519 was a sophisticated political entity that controlled most of central Mexico through a tributary system, with the capital Tenochtitlan — built on an island in Lake Texcoco — among the largest cities in the world.

The Triple Alliance of Tenochtitlan, Texcoco, and Tlacopan had expanded through military conquest from the early 15th century, creating an empire that extracted tribute in goods (cotton, cacao, feathers, precious stones, obsidian) rather than imposing direct administrative control. Conquered territories retained their local rulers and customs as long as they paid tribute and provided military support; resistance was punished with renewed conquest and increased tribute demands.

Human sacrifice in Aztec religion has dominated popular discussion in a way that distorts the overall picture. The sacrificial rituals — which certainly occurred and on a scale that shocked Spanish observers — were embedded in a sophisticated cosmological system in which the gods had sacrificed themselves to create the current world, and humans repaid this debt through ritual bloodshed. The Great Temple at Tenochtitlan, excavated in the 1970s-80s, has yielded evidence of the sacrificial rituals but also of the empire's cosmological ambitions.

The Spanish conquest of 1519–21 was not a simple military victory by a technologically superior force. Hernán Cortés succeeded primarily through political alliance: the Tlaxcalans, traditional enemies of the Aztecs, provided the bulk of the forces that besieged and ultimately captured Tenochtitlan. The smallpox epidemic that swept through the Aztec population in 1520 was perhaps more decisive than any battle.