They actually don't know for sure what led to the decline but a lot of people believe that the Aryans invaded and conquered the Indus Civilization which would make sense because they were the next people group to live there. I hope this helps!
An example of data that a historian would use is "pictorial data", since illustrations can provide unparalleled insight into the past--sometimes even more so than books.
It Is The Lower House Of The Colonial. Virginia Legislature
