There is no clear evidence regarding the cause for the decline of the Harappan culture. Various theories have been postulated
By natural calamities like recurring floods, drying up of rivers, decreasing fertility of the soil due to excessive exploitation and occasional earthquakes might have caused the decline of the Harappan cities
Invasion- according to some scholars, the final blow was delivered by the invasion of the Aryans
The destruction of forts is mentioned in the Rig veda
The discovery of human skeletons huddled together at Mohenjodaro indicates that the city was invaded by foreigners
The Aryans had superior weapons as well as swift horses which might have enabled them to become masters of the region