Mexica tradition held that the people migrated from a homeland called Aztlan, of uncertain location, arriving in the Valley of Mexico as a comparatively late and initially marginal group among older established city-states. Founding legend places the building of Tenochtitlan in 1325 CE on a swampy island in Lake Texcoco, chosen after priests saw an eagle perched on a cactus, an omen still on Mexico's flag today. In 1428, Tenochtitlan joined Texcoco and Tlacopan in the Triple Alliance, which expanded rapidly through conquest and, more often, the threat of it, ruling less as a unified territorial empire than as an overlordship: conquered city-states kept their own rulers but paid heavy tribute in goods, labor and captives to the alliance, Tenochtitlan foremost among its members.
Hernan Cortes landed on the Gulf coast in 1519 and reached Tenochtitlan the same year, exploiting resentment among tributary city-states, above all Tlaxcala, to build a large indigenous coalition against Mexica rule. Smallpox, introduced with the Spanish and devastating to a population with no prior immunity, killed enormous numbers including the emperor Cuitlahuac during the siege that followed, and Tenochtitlan fell in 1521. Colonial Nueva Espana built Mexico City directly atop the ruined Mexica capital. Much of what is known of pre-conquest life comes from later Nahua scholarship undertaken with Spanish friars, above all the Franciscan Bernardino de Sahagun's Florentine Codex, compiled with Nahua informants and illustrators decades after the conquest. Nahuatl-speaking Nahua communities remain a major living Indigenous population in central Mexico today, distinct from the specifically Mexica imperial elite the word "Aztec" most narrowly names.