The Genpei War is the culmination of a decades-long conflict between the Minamoto and Taira clans over dominance of the Imperial court, and by extension, control of Japan.

In the Hōgen Rebellion and in the Heiji Rebellion of earlier decades, the Minamoto had attempted to regain control from the Taira and failed.

The Taira had then begun a series of executions, intended to eliminate their rivals.

In 1177, relations between the Taira clan and the retired Emperor Go-Shirakawa had become highly strained, and the latter had attempted a coup d'état to oust the Daijō Daijin (prime minister), Taira no Kiyomori.

Kiyomori had defeated the former emperor and abolished the Insei system, provoking strong anti-Taira sentiment in so doing.

After the abdication of Emperor Takakura, Taira no Kiyomori, reviving the Fujiwara practice of using the regency to dominate the Japanese government, puts his grandson, Antoku (then only two years of age), on the throne on March 21, 1180.

Go-Shirakawa's son, Prince Mochihito, feels that he has been denied his rightful place on the throne.

Charging Kiyomori with causing suffering, despoiling graves and destroying Buddhist Law, Mochihito supports the Minamoto clan in their conflict against the Taira.

At the same time, Minamoto no Yorimasa leads the Minamoto clan in supporting Mochihito's bid for the Imperial Throne.

In the clashes between the Minamoto and Taira clans that have gone on for decades, Yorimasa had tried to stay out of politics, and had avoided taking sides, although he had participated in the Hogen Rebellion in 1156.

For a time, he had even been friends with Taira no Kiyomori.

During the Heiji Rebellion of 1160, he had leaned just enough in favor of the Taira to allow them to overthrow the Minamoto.

However, by the time he officially retired from military service in Kiyomori's army in 1179, Yorimasa had changed his mind about opposing his own clan.

He has entered the Buddhist priesthood.

In May 1180, he sends out an appeal to other Minamoto leaders, and to temples and monasteries (Enryakuji, Miidera and others) that Kiyomori has offended; he asks for aid against the Taira, in the name of Prince Mochihito.

Kiyomori, in the hope of promoting trade with Song Dynasty China, moves the seat of imperial power to Fukuhara (modern day Kobe) in June, and on the fifteenth of this month, Prince Mochihito flees Kyoto to take refuge in Mii-dera, a Buddhist temple located at the foot of Mount Hiei, in the city of Ōtsu.

Learning of the prince’s flight, Kiyomori sends men after Mochihito, who retreats to Mii-dera, at the foot of Mount Hiei, but discovers that the warrior monks of Mii-dera, for various political reasons, cannot rely on the support of any other monasteries.

Thus, …

…Mochihito flees once more, along with a small Minamoto force, across the River Uji, to the Phoenix Hall of the Byōdō-in.

There they are caught by the Taira forces, and the Battle of Uji is fought.

The bridge is the site of much of the fighting, and the planks are famously smashed to impair the ability of the Taira to cross, but eventually the Minamoto are forced back into the Phoenix Hall.

Yorimasa, suffering defeat at Uji, commits suicide at Byōdō-in.

His ritual suicide by seppuku is the earliest recorded instance of a samurai's suicide in the face of defeat.

Mochihito escapes to Nara, but is captured on his way and killed soon afterwards.