Yoshitoshi became the head of the family in 1580, after his adoptive father, Sō Yoshishige, was defeated, and Tsushima conquered, in a prelude to Toyotomi Hideyoshi's Kyūshū Campaign.

The message the Korean envoys received from Hideyoshi, redrafted as requested on the grounds that it was too discourteous, invited Korea to submit to Japan and join in a war against China.

Shortly after news of the Toyotomi defeat at the Battle of Sekigahara was received by the Joseon Court, a process of re-establishing diplomatic relations was initiated by Tokugawa Ieyasu in 1600.

[7] In 1603, Tokugawa Ieyasu established a new shogunate; and Sō Yoshitoshi was officially granted Fuchū Domain (100,000 koku) in Tsushima Province.

Realizing that the Shogunate would never agree to such a request, Sō Yoshitoshi sent a forged letter and a group of criminals instead; the great need to expel the Ming soldiers pushed Joseon into accepting and to send an emissary in 1608.

These benefited the Japanese as legitimizing propaganda for the bakufu (Tokugawa shogunate) and as a key element in an emerging manifestation of Japan's ideal vision of the structure of an international order, with Edo as its center.