The Aztecs were late comers to the Valley of Mexico, recent emigrants from the desert frontiers of northern Mesoamerica (another term for Central America). Legend says they left their original homeland of Aztlan (hence the name Aztec) around 900 AD. The name Mexico comes from one of the Aztec clans, the Mexica. The early Aztec clans fought against the ruling Toltecs, but were usually defeated. The Mexica instead of fighting the Toltecs fought for them as mercenaries (hired soldiers), and became the military power of the region. In later wars, the Mexica had to flee their Toltec homeland.

Led by their tribal god, Huitzilopochtli, the Mexica Aztecs arrived in the Central Plateau of Mexico around 1200. The city of Tenochtitlan, founded in 1325, was built on a rocky island in Lake Texcoco where the Aztecs discovered an eagle perched on a cactus with a serpent in its mouth. They had been told by Huitzilopochtli that this symbol, still the emblem of modern Mexico, would mark the spot for their capital city. Ancient ideas concerning religion, agriculture, trade, markets, and the need for human sacrifice were adopted by the Aztecs from earlier peoples such as the Toltecs, Mixtecs and Olmecs.

By 1430, Tenochtitlan was part of a three-city league with Tlacopan and Texcoco. The alliance soon gained control of most of central Mexico, except for a few independent cities like Tlaxcala. Internal fighting began, with Tenochtitlan emerging as the centre of a new Aztec Empire. Under the Emperor Moctezuma II (who ruled 1505-1520), Tenochtitlan had a population of 250,000 people making it one of the largest urban centres in the world.

Huitzilopochtli