The origin of Japan's ruling class extends into the mists of ancient history, far before the country was unified after a period of civil wars and well into the first millennium A.D.
Warriors, called bushi, began as the armies of aristocratic factions centered around the Imperial court in Kyoto, but soon eclipsed the nobility entirely to usurp control of the nation, such as it was. The clans they formed fought for supremacy, and in the year 1185, the Minamoto clan defeated the rival Taira clan in the Genpei War.
Shortly thereafter Minamoto no Yoritomo chose for the seat of his military government a place in the east for which a period of Japanese history was later named: Kamakura.