Why did the Mongols move west and south across the steppes? A. Waterways were available in that direction, but not in other directions. B. They wanted to avoid the rugged terrain to the north. C. They sought better land and a more livable climate. D. They wanted to avoid other people who were moving into Mongolia.

From the first explosion of Mongol military might from the steppes of central Asia in the early decades of the 13th century to the death of Timur in 1405, the nomads of central Asia made a last, stunning return to center stage in world history. Mongol invasions ended or interrupted many of the great empires of the postclassical period, while also extending the world network that had increasingly defined the period. Under Chinggis Khan - who united his own Mongol tribesmen and numerous nomadic neighbors into the mightiest war machine the world had seen to that time - central Asia, northern China, and eastern Persia were brought under Mongol rule. Under Chinggis Khan's sons and grandsons, the rest of China, Tibet, Persia, Iraq, much of Asia Minor, and all of southern Russia were added to the vast Mongol imperium. Though the empire was divided between Chinggis Khan's sons after his death in 1227, the four khanates or kingdoms - which emerged in the struggles for succession - dominated most of Asia for the next one and one-half centuries. The Mongol conquests and the empires they produced represented the most formidable nomadic challenge to the growing global dominance of the sedentary peoples of the civilized cores since the great nomadic migrations in the first centuries A. D. Except for Timur's devastating but short-lived grab for power at the end of the 14th century, nomadic peoples would never again mount a challenge as massive and sweeping as that of the Mongols.