Taigen Sessai

Taigen Sessai (1496–1555) was a Zen monk and military strategist who served the Imagawa clan during the Sengoku period. As the moriyaku (guardian-tutor) of Imagawa Yoshimoto, he led a wide range of affairs, from Yoshimoto's education to the clan's diplomacy and military strategy. A high priest of the Rinzai school, he pursued Buddhist training while showing outstanding political and military talent, making him a giant who excelled in both the literary and martial arts.

Sessai was born in Suruga Province (present-day Shizuoka Prefecture) and entered the Buddhist life as a child, undertaking Zen training. He trained at renowned temples such as Kennin-ji and Myōshin-ji, and later became abbot of a temple with ties to the Imagawa family. He had a connection with the Imagawa family from the time of Imagawa Ujichika (Yoshimoto's father), and around Daiei 2 (1522) he was entrusted with raising the young Yoshimoto (Hōgikumaru). From then on, as Yoshimoto's tutor, he taught him Confucian studies, military strategy, and etiquette, while also being deeply involved in the clan's administration.

In military affairs too, Sessai was active as a senior commander of the Imagawa. At the Battle of Azukizaka in Tenbun 17 (1548), he effectively commanded the Imagawa forces and repelled Oda Nobuhide (father of Oda Nobunaga), greatly contributing to Yoshimoto's establishment of his position. In Tenbun 23 (1554), he was also the driving force behind the conclusion of the Kō-Sō-Sun Tripartite Alliance with Takeda Shingen and Hōjō Ujiyasu, securing the Imagawa's northern and eastern flanks. This alliance laid the foundation that allowed Yoshimoto to advance west toward the later Battle of Okehazama.

Sessai died at the age of 60 at Chōkei-ji in Suruga on the 10th day of the intercalary 10th month of Kōji 1 (1555). His achievements are recorded in history as those of the greatest retainer who quietly supported the rise of Imagawa Yoshimoto.