Rise of the Aztecs 

After the fall of Tula, in the 12th century, in the valley of Mexico and surroundings, there were several city states of Nahua-speaking people: Cholula, Huexotzingo, Tlaxcala, Atzcapotzalco, Chalco, Culhuacan, Xochimilco, Tlacopan, etc. None of them was powerful enough to dominate other cities, all of them were proud of their Toltec heritage. Aztec chronicles describe this time as a golden age, when music was established, people learned arts and craft from surviving Toltecs, and rulers held poetry contests in place of wars. 

In the 13th and 14th centuries, around the Lake Texcoco in the Anahuac Valley, the most powerful of these city-states were Culhuacan to the south, and Azcapotzalco to the west. Their rule extended over all the area around Lake Texcoco. 

As a result, when the Mexica arrived to the Anahuac valley as a semi-nomadic tribe, they had nowhere to go. They established themselves temporarily in Chapultepec, but this was under the rule of Azcapotzalco, the city of the "Tepaneca", and they were soon expelled. They then went to the zone dominated by Culhuacan and, in 1299, the ruler Cocoxtli gave them permission to settle in Tizapan, a rocky place where no one wanted to live. They began to acquire as much culture as they could from Culhuacan: they took and married Culhuacan women, so that those women could teach their children. In 1323, they asked the new ruler of Culhuacan, Achicometl, for his daughter, in order to make her the goddess Yaocihuatl. The Mexica sacrificed her. The people of Culhuacan were horrified and expelled the Mexica. Forced to flee, in 1325 they went to a small islet in the center of the lake where they began to build their city "Mexico - Tenochtitlan", eventually creating a large artificial island. After a time, they elected their first tlatoani, Acamapichtli, following customs learned from the Culhuacan. Another Mexica group settled on the north shore: this would become the city of Tlatelolco. Originally, this was an independent Mexica kingdom, but eventually it merged with the islet. 

During this period, the islet was under the jurisdiction of Azcapotzalco, and the Mexica had to pay heavy tributes to stay there. 

Initially, the Mexica hired themselves out as mercenaries in wars between Nahuas, breaking the balance of power between city-states. Eventually they gained enough glory to receive royal marriages. Mexica rulers Acamapichtli, Huitzilihuitl and Chimalpopoca were, in 1372–1427, vassals of Tezozomoc, a lord of the Tepanec nahua. 

When Tezozomoc died, his son Maxtla assassinated Chimalpopoca, whose uncle Itzcoatl allied with the ex-ruler of Texcoco, Nezahualcoyotl, and besieged Maxtla's capital Azcapotzalco. Maxtla surrendered after 100 days and went into exile. Tenochtitlan, Texcoco, and Tlacopan formed a "Triple Alliance" that came to dominate the Valley of Mexico, and then extended its power beyond. Tenochtitlan gradually became the dominant power in the alliance. 

Itzcoatl's nephew Motecuhzoma I inherited the throne in 1449 and expanded the realm. His son Axayacatl (1469) inherited the surrounding kingdom of Tlatelolco. His sister was married to the tlatoani of Tlatelolco, but, as a pretext for war, he declared that she was mistreated. He went on to conquer Matlazinca and the cities of Tollocan, Ocuillan, and Mallinalco. He was defeated by the Tarascans in Tzintzuntzan (the first great defeat the Aztecs had ever suffered), but recovered and took control of the Huasteca region, conquering the Mixtecs and Zapotecs. 

In 1481 Axayacatl's son Tizoc ruled briefly before he was replaced by his younger brother Ahuitzol who had reorganized the army. The empire was at its largest during his reign. His successor was Motecuhzoma II (better known as Moctezuma II), who was emperor when the Spaniards arrived in 1519.