The Kō-Sō-Sun Three-Province Alliance: The 1554 Balance of Power Yoshimoto Built in the Tōkai

In 1554 (Tenbun 23), in the three provinces of Suruga, Kai, and Sagami. The three daimyō Imagawa Yoshimoto, Takeda Shingen (this site's id 5), and Hōjō Ujiyasu concluded an alliance of mutual non-aggression through marriage.

It is called the Kō-Sō-Sun three-province alliance. The heads of the three houses married their sons and daughters to one another, building a balance of power from the Tōkai through the Kantō.

For Yoshimoto, this alliance was one that removed the threats of the Takeda and Hōjō to his rear and opened the way for him to concentrate on his advance to the west.

Takeda Shingen needed to devote his main forces to the pacification of Shinano, and Hōjō Ujiyasu to the management of the Kantō.

All three wished to expand their power in separate directions and wanted to avoid needless attrition with neighboring provinces.

Yoshimoto's aim was Mikawa and Owari to the west, Shingen's Shinano to the north, and Ujiyasu's the Kantō to the east.

That the expansion directions of the three houses did not compete was the premise of the alliance's formation.

The coincidence of interests made possible a long-term alliance among daimyō, rare in the Sengoku period.

It was Yoshimoto's master and strategist Taigen Sessai who led the negotiation of this alliance.

Sessai is said to have mediated among the three houses, making use of the connections and learning he had as a Rinzai monk.

The form of a marriage alliance places the sons and daughters of one's clan in danger should a party betray it.

By taking blood ties as security, the three provinces deterred one another's treachery.

Sessai's diplomatic skill supported Yoshimoto's domain management on both the military and diplomatic fronts.

Having cut off worry to his east and north through the alliance, Yoshimoto secured his rule over Mikawa to the west.

He subordinated the Matsudaira and placed the head's child Takechiyo (the later Tokugawa Ieyasu, this site's id 3) at Sunpu as a hostage.

Yoshimoto's domain reached across the three provinces of Suruga, Tōtōmi, and Mikawa, and his mobilizable strength became the foremost in the Tōkai.

The year 1554, when the three-province alliance was completed, was the year Yoshimoto's power reached its peak.

This margin would turn Yoshimoto toward the invasion of Owari six years later.