The Mexica enter the Valley of Mexico and settle at Chapultepec

After the decline of the Toltecs, Nahuatl-speaking groups from the north entered the Valley of Mexico, all of them, according to their traditions, from a homeland called Aztlan. The Mexica, whose tutelary god Huitzilopochtli had told them to part from the other "Aztec" tribes and take a new name, came among the last. They found the valley already divided among city-states, the strongest of which were Culhuacan in the south, whose rulers claimed Toltec descent, and Azcapotzalco of the Tepanecs in the west. One reconstruction places their arrival in the north of the valley, around Xaltocan and Tzompanco (Zumpango), in 1226–1227.

From there they moved to Chapultepec, "grasshopper hill", on the western shore of Lake Texcoco — a place of springs where, in Aztec legend, the last Toltec king, Huemac, had died. Five chronicles give dates for this settlement which, as correlated by Michael E. Smith — in several cases by moving them forward one 52-year calendar cycle — fall between 1245 and 1257, on average about 1248; another reconstruction places it around 1280. Tradition sets at Chapultepec the choice of Huitzilihuitl the Elder as the Mexica leader and the sacrifice of Copil, an episode that foreshadows the founding of Tenochtitlan.

In 1299 — the year given by the Crónica Mexicayotl — Azcapotzalco, Culhuacan, Xochimilco, Xaltocan and other neighbours joined forces to drive the Mexica out: Huitzilihuitl and his family were captured and put to death, and the survivors hid in the reed beds of Acocolco before going to Culhuacan. Its ruler Coxcoxtli allowed them to settle as vassals on the barren land of Tizapan, south-west of Chapultepec, where they came under Culhua influence — the first Mexica tlatoani, Acamapichtli, was a grandson of Coxcoxtli. After a clash with the Culhua they moved on, and in 1325 (by another calculation, 1345) they founded Tenochtitlan on an island in the lake. Chapultepec, with its springs, later supplied Tenochtitlan with fresh water through an aqueduct.