The Aztecs were a race of nomadic warriors who migrated into the Valley of Mexico during the 12th century. The Aztecs took on some aspects of the Toltec civilization they encountered, and in 1325 they built their capital, Tenochtitlan, on two islands in Lake Texcoco. At first a group of elders ruled, but towards the end of the 14th century they elected their first king: Tennoch (1349-93).
Around 1430 the 3-city league of Tenochtitlan- Tezcuco-Tlacopan was formed to gain political control. In a series of bloody campaigns the league subjugated neighboring tribes and made them vassals until the empire stretched from the Gulf of Mexico to the Pacific Ocean.
The Aztecs had to put down repeated revolts of the tribes in the outlying areas, an effort which drained the strength of the kingdom and weakened its capacity to resist the European invaders in 1519.