The Indus Valley Civilization flourished in antiquity before recorded history until their decline around 1900 BCE.[13] Punjab was enriched during the height of the Vedic period, but declined in predominance with the rise of the Mahajanapadas.[14] The region formed the frontier of initial empires during antiquity including the Alexander's and Maurya empires.[15][16] It was subsequently conquered by the Kushan Empire, Gupta Empire,[17] and then Harsha's Empire.[18] Punjab continued to be settled by nomadic people; including the Huna, Turkic and the Mongols.