What happened c.1500 BCE?

Late Harappan
Around 1800 BCE, signs of a gradual decline began to emerge, and by around 1700 BCE, most of the cities were abandoned. However, the Indus Valley Civilisation did not disappear suddenly, and many elements of the Indus Civilization can be found in later cultures.
A possible natural reason for the IVC's decline is connected with climate change: The Indus valley climate grew significantly cooler and drier from about 1800 BCE.
The region lies on the ancient route used by successive waves of migrations from Aryans to Huns, and later by Turks and Mughals to South Asia over the passes in the Hindu Kush.