The word Aztlan might be the root of the term that identifies the Aztecs. According to sources such as the Migration Scrolls and Aztec codex, Aztlan was the place where Chicomostoc, or place of the seven caves, was located. It was a complex of seven temples where seven tribes of Aztecs worshiped Huitzilopochtli, the god of war. These tribes had a common root known as the Nahuatlaca people. They were the Xochimilca, the Tlahuica, the Acolhua, the Tlaxcalan, the Tepaneca, the Chalca, and the Mexica. Apparently, worshiping Huitzilopochtli together with the tyranny of Mexica Chicomoztoca elite in Aztlan, motivated Aztecs to migrate south to find the "promise land". They believed they were "the chosen people" by the god of war.
During their journeys, Huitzilopochtli, through dreams, revealed to the Aztecs led by their priest Tenoch, the place where they had to establish; this place was going to be recognized through the presence of a small island where an eagle, on a cactus, would have been eating a snake.
Nonetheless, by that time, the Valley of Mexico was not unpopulated. Several different cultures had established and developed previously on these terrains, characterized by the presence of lakes. Some of these cultures were Chalco, Xochimilco, Tlacopan, Culhuacan, and Azcapotzalco. Therefore, when Aztecs arrived, they had no place where to settle. From sources like the Boturini Codex, the Aubin Codex, as well as from legends, it is known that they first settled, around 1248, in Chapultepec, a hill in the west shore of the Texcoco Lake; and later, in 1299 in Tizapan.
The people of Culhuacan allowed Aztecs to settle there and, as a result, Mexicas or Aztecs assimilated to this culture. Later, due to the ferocious and cruel "cultural" practices of the Aztecs, they were expelled and forced to flee. At that point, they moved out to Texcoco Lake where finally they found the small and swampy island with the eagle on a cactus eating a snake. Thus, Tenochtitlan, the capital of the Aztec Empire, began to be built.
During the early years of Tenochtitlan, resources were limited and neighbors were not pleasant with the new presence. But the times of eating snakes and vermin were going to pass as Aztecs developed a farming system called "chinampas" that proved to be highly efficient; they built thatches made of mud and temples of "tezontle" stones. By the second half of the XIV Century, Aztecs started to increase their "military" power letting their warrior spirit to flow. Hence, they attacked and subdued their neighbors.
Tributes, payments, and prisoners of war brought to sacrifice were common practices that account for the expansion and force of the Aztec Empire. Hatred was a usual feeling among neighboring cities. Gradually, the empire began to take form.
After Tenoch, Acamapichtli ruled until 1395 and then, he was followed by Huitzilihuitl. Ten more tlatoanis ruled Tenochtitlan thereafter during the existence of the empire. Moctezuma II was ruling the city by the arrival of the conquerors led by Hernan Cortez in 1519. Then, Cuitlahuac, his younger brother, became the king. However, Cuauhtemoc was the last Aztec emperor.