The Isshiki Clan (Isshikiuji) (一色氏)
The Isshiki clan comprised a samurai family. The original surname was Genji (Gen clan). The family was one of the branch families of the Ashikaga clan, a distinguished family among the Kawachi-Gen clan, which was one of the Seiwa-Genji (Seiwa Gen clan). It was one of the Shishiki families (four administrators' families: Yamana, Isshiki, Akamatsu, Kyogoku) of the Muromachi bakufu (Japanese feudal government headed by a shogun) in the Muromachi era.
The originator was Koshin ISSHIKI, a son of Yasuji ASHIKAGA. Because the family was started in Kira no sho Isshiki in Mikawa-no-kuni (present-day Isshiki-cho Hazu-gun Aichi Prefecture), they gave themselves the surname Isshiki.
There is Isshikida as the same surname but with a different pronunciation.
However, the family of Norimitsu Isshiki, the second child of Noriuji, rendered distinguished service; therefore, the family power was regained to become shugo (guard feudal lord) of Mikawa-no-kuni, Wakasa-no-kuni and Tango-no-kuni, and also held the reins of power as one of the Shishiki families, in what was the most flourishing time for the Isshiki clan. However, since they had gained too much power, Yoshitsura Isshiki, who was the family head of the Isshiki clan, was killed by Yoshinori ASHIKAGA, the sixth-generation shogun of the Muromachi, and an attack by the Takeda clan resulted in the weakening of family power. Then, Yoshinao ISSHIKI gained the confidence of the eighth-generation shogun, Yoshimasa ASHIKAGA, but in the turmoil of the Onin War in 1467 the Isshiki clan belonged to the Western army led by Sozen YAMANA because the clan was opposed to Wakasa-Takeda clan, which belonged to the Eastern army led by Katsumoto HOSOKAWA; this resulted in the defeat of the Western army, and thereafter the clan gradually weakened.
During the Sengoku period, while the conflict with the Wakasa-Takeda clan continued, rebellions and gekokujo (upheavals wherein the inferior defeats the superior) frequently occurred in the area, whereby the power of the Isshiki clan became further weakened. In the generation of Yoshimichi ISSHIKI, Yoshimichi was killed in 1579 as a result of the invasion of an army led by Yusai HOSOKAWA, as ordered by Nobunaga ODA, and thereafter Yoshimichi's son Yoshisada ISSHIKI and Yoshimichi's younger brother, Yoshikiyo ISSHIKI tried hard to fight against the Hosokawa army; however, they were killed in 1582, and this resulted in the complete downfall of the Isshiki clan in Tango.
Incidentally, the Isshiki clan families were scattered around the country, and in Kanto there existed the Satte Castellan Isshiki clan (a family of Osakane ISSHIKI, who was a grandchild of Naouji ISSHIKI) as a family of the Kamakura Kubo, and they served Koga Kubo till the end; moreover, the clan continued as the hatamoto (direct retainer of the shogun) and the domain of Mito during the Edo period. The Isshiki-Tsuchiya clan served the Takeda clan, the guard of Kai-no-kuni, during the Sengoku period, and after the downfall of the Takeda clan Tadanao TSUCHIYA was employed by Ieyasu TOKUGAWA to become the lord of the domain of Kururi, Kazusa-no-kuni. Ujitsugu NIWA, of the Isshiki-Niwa clan, who served both the Oda and Toyotomi clans, became the originator of the domain of Iho, Mikawa-no-kuni, during the Edo period.
However, Nagahide NIWA and his family were of the Taira clan, and the family line was therefore different (categorized under the Niwa clan). Nagahide's grandchild, Mitsuhide NIWA, became the lord of the domain of Nihonmatsu, Mutsu-no-kuni.