Lifespan: Eiroku 11 (1568) to 4/7 of Tenshō 18 (1590)
Lord: Date Masamune
Father: Date Terumune
Mother: Yoshihime (daughter of Mogami Yoshimori)
Siblings: Masamune, Kojirō, Hideo, Senshihime
Date Masamichi served as a bushō during the Azuchi-Momoyama period (and possibly the early Edo period). Masamichi was the second son of Date Terumune (a sengoku daimyō and the sixteenth head of the Date clan). Masamichi was commonly known as Kojirō and there are no historical records from that era to substantiate adoption of the name Masamichi.
Masamichi was born around 1568 as the second son of Date Terumune. His mother was Yoshihime, the daughter of Mogami Yoshimori. Yoshihime was also the mother of Masamichi’s older brother – Date Masamune. His grandfather, Date Tanemune, was the grandchild from a daughter married into another family so Masamichi was the great-great-grandchild of Ashina Moritaka.
His mother, Yoshihime, adored Kojirō more than his older brother, Masamune, who was in conflict with the Mogami clan. Within the Date clan, a power-struggle likely occurred between Masamune and Kojirō but, in 1584, Terumune assigned the role as head of the clan to Masamune.
Masamune’s succession occurred the same month that Ashina Moritaka was assassinated. There is a letter noting a consultation between the Date and Ashina by which Terumune pledged to Ashina Moriuji (Moritaka’s father) to send his second son (Kojirō) after he matures to serve the Ashina clan. There is another record stating that, just before the Conquest of Odawara, Masamune noted to the Toyotomi administration that he sought to subdue the Ashina owing to the breach of a promise to his father, Terumune, to have Masamune’s younger brother (Kojirō) become the head of the Ashina, while Kojirō was instead later adopted by the Satake clan despite a discussion just after the assassination of Moritaka about having Kojirō become the head of the Ashina clan. Owing to interference by the Satake clan, the failure to have Kojirō become Moritaka’s successor may have been a factor in the replacement of Terumune by Masamune as the head of the Date clan.
Terumune died the year after Ashina Moritaka so was not involved in later succession struggles.
In 1587, after a succession problem rose again in the Ashina clan, Kojirō was recommended to become the next head of the Ashina clan, but, owing to the devices of Kanagami Moriharu (the regent of the Ashina clan), Ashina Yoshihiro (the son of Satake Yoshishige) became the next head of the clan.
On 7/14 of 1587, Kojirō, together with his older brother, Masamune, departed to the Shifuku Temple in Natsukari to visit the tomb of their father. The pair then returned to Yonezawa Castle. Yonezawa was a flatland castle in the Okitama District of Dewa Province. During the Tenbun Conflict (1542 to 1548) between Date Tanemune and his son, Date Harumune (the grandfather of Masamune and Kojirō), Harumune moved from Koori-Nishiyama Castle in Mutsu Province to Yonezawa in Dewa. The pair then visited their mother at the Ohigashi residence.
In 1590, Masamichi suddenly died. It is said that he was killed by Masamune as a result of their sibling rivalry. According to theories based on documents from the Edo period, Yoshihime called upon Masamune for him to participate in the Conquest of Odawara led by Toyotomi Hideyoshi, and while coming to visit his mother, Yoshihime and Masamichi tried to poison him. For this reason, he killed Masamichi, while Yoshihime took responsibility for the attempted murder by taking her own life. However, owing to the date of the source, this is considered a fabricated story so many questions remain in regard to the cause of death of Masamichi.
According to archives at the Daihigan Temple, the abbot at the time was Date Hideo – the last son of Date Terumune, which may have been a reference to Masamichi. If this is correct, then he would have died on 8/21 of 1642.