In 1428, under their leader Itzcoatl, the Aztecs formed a three-way alliance with the Texcocans and the Tacubans to defeat their most powerful rivals for influence in the region, the Tepanec, and conquer their capital of Azcapotzalco. Itzcoatl’s successor Montezuma (Moctezuma) I, who took power in 1440, was a great warrior who was remembered as the father of the Aztec empire. By the early 16th century, the Aztecs had come to rule over up to 500 small states, and some 5 to 6 million people, either by conquest or commerce. Tenochtitlán at its height had more than 140,000 inhabitants, and was the most densely populated city ever to exist in Mesoamerica.
Bustling markets such as Tenochtitlan’s Tlatelolco, visited by some 50,000 people on major market days, drove the Aztec economy. The Aztec civilization was also highly developed socially, intellectually and artistically. It was a highly structured society with a strict caste system; at the top were nobles, while at the bottom were serfs, indentured servants and slaves.