Although fighting men played a prominent role in Japanese history from the nation's beginning, the class of warriors known as samurai did not emerge until the twelfth century A. D. By that time, many small chiefdoms had been unified into a central state headed by an emperor or empress who was believed to be divine. The imperial court was located at Heian-kyo (now Kyoto ), where the reigning monarch lived in a splendid palace, surrounded by men and women of noble birth.

Despite the prosperity of the court, unification of Japan was by no means complete. Rebellious warlords still challenged the authority of the emperor periodically, and fierce bands of Ainu (the original natives) still resisted subjugation. However, the good life of the imperial court was so far removed from such matters that emperors increasingly neglected to provide adequate protection for...