What are ways in which the relationship between nomads and civilized people shaped historical developments in regards to the nomadic pastoralists being a creative force in world history from 2000 B.C.E. – 1500 C.E.?
World history tends to emphasize the role of complex, settled civilizations. However, the nomadic way of life persisted throughout world history down to 1500CE and beyond.
The Huns, coming from the central Asian steppes, served as a creative force and shaped historical developments by helping to force other people groups into the sphere of Roman influence and territory. The Goths and Vandals were pushed into Europe by the advancing Huns, and ended up destroying much of the Mediterranean world's infrastructure. The Germanic tribes which followed the Goths' advance into Spain helped to create the Frankish kingdoms and also sailed to the British Isles, while the Vandals' rampage through North Africa created a power vacuum which was filled eventually by first the Byzantines, then the emergine caliphates of the Islamic conquests. The Huns themselves disappeared or merged with other nomads.
Another people group to be considered is the Mongols. After obliterating the Jin Dynasty of northern China and learning how to effectively use siege weapons against walled cities, the Mongols' Great Ride across the myriad kingdoms of central Asia wiped many of the kingdoms which had thrived. Advancing into Russia, the Mongols ended the Kievan-Russian state's importance and prepared Muscovite Russia to determine the course of Russian history. Moscow bided its time, then threw off the rule of the Golden Horde and put an end to the western khanate of the Mongol empire. China also waited for several decades before throwing off Mongol rule and establishing the Ming Dynasty.
In each case, the movement of these mounted nomadic peoples served as a catalyst for change in cultural and political institutions.