The continents of Europe and Asia share the largest plain in the world, a vast steppe, or grassland, that stretches five thousand miles from Hungary in the west to Manchuria in the east. From at least 4000 B. C., tribes of nomadic herders have struggled to wrest a living from this dry, flat wilderness. Relations between the tribes were volatile. At times, they formed alliances and lived together in harmony. Far more often, though, they were at war with each other, battling bitterly for control of grazing land to feed their animals.

The people who came to be known as the Mongols migrated into this cauldron of tribal strife during the eighth century. By the time Genghis Khan was born in the middle years of the twelfth century, conflict had become so intense and so frequent that the tribes were threatening to destroy...