Aristocrats had been strengthening their power since the Nara Period, and in the Heian Period they expanded it further.
Among them, the Fujiwara clan gained overwhelming influence by placing their descendants on the throne.
The key figure was Fujiwara no Yoshifusa.
Through political maneuvering, Yoshifusa had his nephew, Prince Michiyasu, named crown prince, becoming the emperor’s maternal relative and securing strong authority.
He then used that power to drive out rival nobles.
He also sent his daughter into the imperial consort system.
When Prince Korehito—later Emperor Seiwa—was born to Prince Michiyasu and Yoshifusa’s daughter, Yoshifusa became the first non-imperial Daijo-daijin (Chancellor of the Realm), and the following year rose to regent (sessho).
After Yoshifusa, the Fujiwara continued to monopolize key posts such as regent and kampaku (chief advisor).
It’s fair to say the Heian Period marked the peak of Fujiwara regency politics.
A famous poem by Fujiwara no Michinaga—who served as regent at the height of Fujiwara power—captures the era: “There is nothing in this world that does not go as I wish; like the full moon, lacking nothing, all is complete.”