Ancient Greeks had a word for the people who lived on the wild, arid Eurasian steppes stretching from the Black Sea to the border of China. They were nomads, which meant “roaming about for pasture.” They were wanderers and, not infrequently, fierce mounted warriors. Essentially, they were “the other” to the agricultural and increasingly urban civilizations that emerged in the first millennium B.C. As the nomads left no writing, no one knows what they called themselves. To their literate neighbors, they were the ubiquitous and mysterious Scythians or the Saka, perhaps one and the same people. In any case, these...