The Fujiwara were the most powerful clan during
Japan's
Heian era. Their power was gained through careful political maneuvering, especially a large number of
political marriages between them and the royal family. Eventually, the emperor lost all real power, and all actual decision-making positions were occupied by Fujiwara.
However, the Fujiwara became so preoccupied with court machinations and political games which were not supported by real, military power, that they were eventually superseded by the Taira clan.