The Aztecs at first lived under the dominion of Azcapotzalco, the mightiest of the city-states in the valley of central Mexico. They helped this town to conquer territory in the early 15th century, but in 1 428 they defeated Azcapotzalco itself with the assistance of allies. By 1431, Tenochlitlan had become an independent state in alliance with the neighboring citles of Texcoco and TIacopan. This triple alliance soon controlled the entire valley and, with Tenochtltlan as the dominant partner, began a program of military expansion that was still unfinished at the time of the Spanish conquest.
Montezuma I (r. 1440-68), grandfather of MONTEZUMA II, expanded the Aztec empire until It stretched from the Atlantic to the Pacific coasts. A total of 489 cities paid tribute to the alliance. From these conquered cities came foodstuffs to feed the growing population of Tenochtitlan, together with luxury goods for Aztec nobles and warriors. They also received exotic raw materials lacking In the Aztec homeland-gold, copper, tropical feathers, gemstones, rubber, jade, amber, jaguar skins, and chocolate. Conquest also furnished war captives for sacrifice to the gods.
The alliance powers made no attempt to unify the area they controlled or to change the customs of conquered peoples. The empire was held together by force rather than by loyalty, and the subject states were eager to shake off Aztec control. With the arrival (1 519) of the Spanish army of Heman Cortes, several Mexican cities willingly joined forces with the invaders. These Indian allies, in particular the Tlaxcalans (hereditary enemies of the Aztecs), were an essential element in the conquest of Mexico, supplying auxiliary troops, food, transport, and secure bases in friendly territory. In 1521, Tenochtitlan, under the leadership of CUAUHTEMOC, capitulated to Cortes, and the days of Aztec dominion ended.