In addition to experiencing such destabilising factors such as climate shifts and over-cultivation of land, the area was also vulnerable to attack from nomadic peoples. Following the Mongol and Timurid invasions and subsequent massacre of Assyrians, it was left with only a scant permanent population. In the centuries that followed, a number of nomadic and semi-nomadic Arabic- and Kurdish-speaking tribes wandered the area with their livestock into the 20th century, when most of them were forced to settle by governmental policies.