The Aztec Empire, also known as the Triple Alliance or historiographically as the Tenochca Empire, was an alliance of three Nahua city-states: Mexico-Tenochtitlan, Tetzcoco, and Tlacopan.  These three city-states ruled that area in and around the Valley of Mexico from 1428 until the combined forces of the Spanish conquistadors and their native allies who ruled under Hernán Cortés defeated them in 1521.  Its people and civil society are historiographically referred to as the Aztecs or the Culhua-Mexica.

The alliance was formed from the victorious factions of a civil war fought between the city of Azcapotzalco and its former tributary provinces.  Despite the initial conception of the empire as an alliance of three self-governed city-states, the capital Tenochtitlan became dominant militarily.  By the time the Spanish arrived in 1519, the lands of the alliance were effectively ruled from Tenochtitlan, while other partners of the alliance had taken subsidiary roles.

The alliance waged wars of conquest and expanded after its formation.  The alliance controlled most of central Mexico at its height, as well as some more distant territories within Mesoamerica, such as the Xoconochco province, an Aztec exclave near the present-day Guatemalan border.  Aztec rule has been described by scholars as hegemonic or indirect.  The Aztecs left rulers of conquered cities in power so long as they agreed to pay semi-annual tribute to the alliance, as well as supply military forces when needed for the Aztec war efforts.  In return, the imperial authority offered protection and political stability and facilitated an integrated economic network of diverse lands and peoples who had significant local autonomy.