Tenochtitlan, the capital of the Aztec empire, was founded by the Aztec or Mexica people around 1325 C.E. According to legend, the Mexica founded Tenochtitlan after leaving their homeland of Aztlan at the direction of their god, Huitzilopochtli. Huitzilopochtli directed them to build where they saw an eagle perched on a cactus, eating a snake. When they saw this exact scene on an island (located in what was once Lake Texcoco), they interpreted it as a sign from their god and founded Tenochtitlan on that island. Read more
Source: Ancient History Encyclopaedia