The Mexica Triple Alliance or Aztec Empire began as an alliance of three Nahua city-states or "altepetl": Tenochtitlan, Texcoco, and Tlacopan. These city-states ruled the area in and around the Mexico Valley from 1428 until they were defeated by the combined forces of the Spanish conquistadores and their native allies under Hernán Cortés in 1521.
The Triple Alliance was formed from the victorious faction in a war between the city of Azcapotzalco and its former tributary provinces. Despite the initial conception of the empire as an alliance of three cities, Tenochtitlan quickly established itself as the dominant partner. By the time the Spanish arrived in 1520, the lands of the Alliance were effectively ruled from Tenochtitlan, and the other partners in the alliance had assumed subsidiary roles.
The alliance waged wars of conquest and expanded rapidly after its formation. At its height, the alliance controlled most of central Mexico as well as some more distant territories within Mesoamerica. Aztec rule has been described by scholars as "hegemonic" or "indirect". Rulers of conquered cities were usually left in power as long as they agreed to pay semi-annual tribute to the alliance or provided military support in wars with enemy states.