Lifespan: Daiei 7 (1527) to 4/16 of Tenshō 15 (1587)
Titles: Junior Fifth Rank; Vice Minister of Military Affairs
Lord: Date Tanemune → Date Harumune → Date Terumune → Date Masamune
Father: Date Tanemune
Mother: Daughter of Nakajō Sadasuke
Siblings: Harumune, Ōsaki Yoshinobu, Sanemoto, Genbamaru, Sōchō, Koori Munesada, Kasai Harukiyo (adopted by Kasai Harushige), Yanagawa Munekiyo, Murata Munetane, Gokurakuin Sōei, Watari Tsunamune, Watari Motomune, Ōari Yasutoshi, Shichirō
Wife: [Formal] Shōseiin (second daughter of Date Harumune)
Date Sanemoto served as a bushō during the Sengoku and Azuchi-Momoyama periods. Sanemoto was the lord of Ōmori Castle in the Shinobu District of Mutsu Province. Originally, his father, Date Tanemune, planned for him to be adopted by the Uesugi clan, the military governors of Echigo Province, to become the successor to Uesugi Sadazane. This plan, however, failed owing to internal opposition that escalated into the Tenbun Conflict. Thereafter, he served as the leader of the family members remaining in Oushū. Later, he became the founder of the Watari-Date, a cadet family of the Date clan based in the Watari District of Mutsu.
In 1527, Sanemoto was born as the third son of Tanemune, the sengoku daimyō of Mutsu. His childhood name was Tokimunemaru. The Date clan of Mutsu Province had connections to the Uesugi clan and to Echigo Province. Tokimunemaru’s grandmother on his father’s side hailed from the Uesugi, while his mother was the younger sister of Nakajō Fujisuke, lord of Tossaka Castle in Echigo.
In the summer of 1542, in a bid to further expand his domain, his father, Tanemune, planned to send Tokimunemaru for adoption by Uesugi Sadazane, the military governor of Echigo Province. Prior to the adoption, Tokimunemaru received one of the characters in his name from Sadazane, and thereafter became known as Date Sanemoto. He also received a sword from a famous sword-maker named Usami Sadamitsu inscribed with a motif of the family crest of the Uesugi clan featuring a ring of bamboo surrounding sparrows, symbolizing a perfect match.
Tanemune planned for an elaborate entourage of 100 elite mounted soldiers to accompany Sanemoto on his move to Echigo. Meanwhile, Sanemoto’s older brother, Date Harumune, and older brother-in-law, Sōma Akitane, opposed the implications of the plan for the Date vis-à-vis the Uesugi, whereupon they conspired with Nakano Munetoki and Koori Kagenaga (who harbored their own grievances toward Tanemune over his policies). Under this plan, the men incarcerated Tanemune in Koori-Nishiyama Castle and prevented Sanemoto from being adopted by the Uesugi.
Tanemune, however, escaped from the castle with the assistance of Koyanagawa Munetomo, whereupon he directed troops toward Harumune and Akitane, triggering the Tenbun Conflict that enveloped the entire southern region of Ōu (meaning both Mutsu and Dewa provinces) in northeast Japan. Sanemoto joined the side of Tanemune, and despite fighting valiantly in the Shindachi region, after the conflict ended in victory for Harumune, he surrendered and was pardoned. At the same time, the faction in Echigo opposed to Sanemoto becoming the heir to the Uesugi prevailed over those in favor, so the plan for Sanemoto to become the heir of Sadazane was vanquished.
After the conflict, and following Harumune’s move to Yonezawa Castle, Sanemoto wed Harumune’s second daughter and became the lord of Ōmori Castle, taking responsibility to govern the Shindachi region in lieu of Harumune. Sanemoto’s territory encompassed thirty-one villages in the Shinobu District and two villages in the Natori District. When his nephew, Date Terumune, became head of the family, battles broke out against the Tamura and Sōma clans, while smaller daimyō families in the area such as the Ōshū-Hatakeyama and Ōuchi came under the governance of the Date.
In 1583, Sanemoto transferred control of the clan to his eldest son, Date Shigezane, retired in Hattchōme Castle, and assumed the name of 棲安斎. After his retirement, Sanemoto continued to serve as a clan elder, engaging in diplomacy and strategy, and, together with Shigezane, frequently rescued Date Masamune from precarious situations.
Sanemoto died in Hattchōme Castle on 4/16 of Tenshō 15 (1587).