The Minamoto clan (源, Minamoto-shi) was one of the four clans that dominated the politics of Japan during the Heian era, the other three being the Fujiwara, Taira and Tachibana.
"Genji" (源氏) is the other name of the Minamoto clan, based on the alternate pronunciation of the Chinese characters for Minamoto (gen) and ji, or "clan."
"Minamoto" was an honorary name given by emperors of the Heian period (794-1185) to their sons and grandsons after accepting them as royal subjects.
According to tradition, to facilitate imperial succession and prevent rivalry for the throne, princes who were ineligible or excluded from the succession were given a title and became subjects of the emperor.
The Minamoto, like their rivals the Taira, came from this tradition. Emperor Saga, who reigned from 809 to 823, granted in 814 the kabane Minamoto no Ason to 33 of his 50 children who could not inherit the throne.
As a result, they and their descendants ceased to be part of the imperial family, thus creating the Saga Genji lineage. Later, Emperors Seiwa, Murakami, Uda and Daigo, among others, also gave this name to their sons, creating the various lineages bearing their names followed by "Genji".
Only one Minamoto returned to the imperial succession: Minamoto no Sadami, who became in 887 the emperor Uda, and thus created a particular branch of the Minamoto clan.
Of the various Minamoto lineages, the most important is the Seiwa Genji lineage, descended from Minamoto no Tsunemoto, a grandson of the 50th emperor, Seiwa.
Tsunemoto received the patronymic of Minamoto in 961, moved to Kantō province, and founded one of the major warrior dynasties there.
Later, his son Minamoto no Mitsunaka formed an alliance with the Fujiwara, and thereafter the Fujiwara often called upon the Minamoto to restore order in the capital, Heian-kyō (later renamed Kyoto.
Mitsunaka's eldest son, Minamoto no Yorimitsu (948-1021), became the protégé of Fujiwara no Michinaga, and another of his sons, Yorinobu (968-1048), put down the rebellion of Taira no Tadatsune in 1032.
Yorinobu's son Minamoto no Yoriyoshi (998-1075) and his grandson Minamoto no Yoshiie (1039-1106) pacified most of northeast Japan between 1051 and 1087.
The fate of the Seiwa Genji declined during the Hōgen Rebellion (1156), when the Taira executed almost the entire line. During the Heiji Rebellion in 1160, the head of the Seiwa Genji clan, Minamoto no Yoshitomo, died in battle.
Taira no Kiyomori took control of Kyōto by allying with the retired emperors Shirakawa and Toba and infiltrating the kuge. He sent Minamoto no Yoritomo, Yoshitomo's third son, into exile, along with his half-brother Minamoto no Noriyori.
Minamoto no Yoshitsune, another half-brother of Yoritomo, was placed in a monastery, while all the other child heirs of the Minamoto clan were executed.
In 1180, the old Minamoto no Yorimasa and Prince Mochihito called for arms against the Taira, and the Genpei War began.
After their deaths at the beginning of the war, Yoritomo, along with his half-brothers and his cousin Minamoto no Yoshinaka, answered the call and joined the fight against Taira rule.
The Genpei War lasted five years and resulted in the destruction of the Taira. In 1192, he received the title of Seii tai-shogun and established the first bakufu in Kamakura.
Thus, the Seiwa Genji lineage proved to be the strongest Minamoto lineage during the late Heian era, with the formation of the Kamakura shogunate.
Moreover, it is from the Seiwa Genji lineage that the future Ashikaga (who gave the Muromachi shoguns), Nitta and Takeda clans came.