The rigors of their economic existence had turned the Mongols from hardy herders into natural soldiers, but before Genghis Khan could weld them into a fighting force to carry out his imperial dreams he had to overcome one monumental obstacle: The warring tribes of the steppe were not a unified nation. The task of unification was enormous because the hatred between the tribes sometimes went back generations and avenging past wrongs was a matter of honor.

The society into which Genghis was born was hierarchical. At the bottom were individual families. These were joined together in clans, which in turn made up tribes. However, two aspects of Mongol life clouded this relatively simple picture. The first of these was exogamy; that is, a man was required to marry outside his own tribe. The second was polygamy; a man could have as many...