When and why did the Aztec migrate south?

The Aztecs migrated south to central Mexico in the 12th century, seeking a new homeland after leaving their mythical birthplace of Aztlan. They settled in the Valley of Mexico around 1325, founding their capital city of Tenochtitlan on an island in Lake Texcoco. This migration was driven by a combination of push factors like famine and drought in their previous territories and pull factors such as the fertile land and resources of the Valley of Mexico.