Sō clan In 1587 , Toyotomi Hideyoshi confirmed the Sō clan possession of Tsushima . In the struggles which followed the " Taiko " ' s death , the clan sided with the Tokugawa ; however , they did not participate in the decisive battles which preceded the establishment of the Tokugawa shogunate . The descendants of " tozama " Sō Yoshitoshi ( 1568 - 1615 ) remained at Tsushima - Fuchū Domain ( 100 , 000 koku ) in Tsushima Province until the abolition of the " han " system . The head of this clan line was ennobled as a " Count " in 1884 . History . Historians consider the Sō clan to have been an offshoot of the Koremune clan ( 惟宗氏 ) , who served as local officials of Dazaifu and Tsushima Province . The earliest evidence of Sō clan cohesion arises in the 11th century . The Koremune had their start as governors of Tsushima following an incident in 1246 , when the Abiru clan , local district officials ( " zaichōkanjin " ) on Tsushima , rebelled against the Chinzei Bugyō and the Dazaifu government which governed all of Kyūshū and the surrounding regions on behalf of the Kamakura shogunate . Dazaifu ordered Koremune Shigehisa to stop the rebellion and to destroy the Abiru clan . He was rewarded for his victory with the post of " Jito " ( local land steward ) , by the Shōni clan , " shugo " ( governors ) of Tsushima . The Koremune extended its influence on Tsushima over the course of the Kamakura period , as the deputies of the Shōni . When the Mongols invaded Japan in 1274 , clan head Sō Sukekuni fought against the invaders and died on Tsushima . The Sō clan fought for the Shōni clan and for the Ashikaga Northern Court during the Nanboku - chō period ( 1336 - 1392 ) , and seized a portion of Chikuzen Province . Imagawa Ryōshun became " Chinzei Tandai " ( head of the Dazaifu government ) soon afterwards , and the Imagawa clan became " shugo " of Tsushima . When Imagawa Ryōshun was dismissed from his post in 1395 , Sō Sumishige became " shugo " of Tsushima . Though now holding the hereditary post of " shugo " of Tsushima , the clan remained vassals of the Shōni until the late 15th century . In the course of breaking away from the Shōni clan , the Sō clan started to claim that it originated with a grandson of Taira no Tomomori , Taira no Tomomune . The Sō clan moved its base from northern Kyūshū to Tsushima around 1408 . Although it struggled to keep its territory in Chikuzen on Kyūshū , the clan was finally purged from that region by the Ōuchi clan in the mid - 15th century . From roughly the 1430s to the 1550s , the clan worked towards independence , its sights set on gaining true control of Tsushima , and on establishing friendly relations with Joseon Dynasty Korea . The first of these goals was attained through a reversal of fate in which the Shōni came to rely upon the Sō for military aid against the Ōuchi . Sō Sadamori and his younger brother Sō Morikuni helped to defeat the Ōuchi over ten times , and in doing so deepened their clan ' s relationship with the traders and merchants of Hakata , one of Kyūshū ' s major ports . Being based on the mountainous island , the Sō clan heavily relied on trade with Korea . Fortunately , the Sō clan shared mutual interests with the Joseon Dynasty . In 1443 , the Daimyo of Tsushima , proposed a Gyehae treaty . The number of trade ships from Tsushima to Korea was decided by this treaty , and the Sō clan monopolized the trade with Korea . The Sō were required to greatly reduce the number of pirate attacks on Korea , and in return would be granted a virtual monopoly on Japanese trade with their neighbors on the peninsula . This would later lead to the emergence of the Sō as one of the major clans in Japan , their wealth and power from trade more than making up for their small territory and submissive position vis - a - vis Korea . Alongside the Shōni clan , whose hereditary clan heads now regularly operated under Sō clan guidance , the Sō fought the Ōuchi numerous times across the Sengoku period ( 1467 - 1600 ) , and later the Mōri and Ōtomo clan s as well ; the clan lost and regained their territory in Chikuzen province on Kyūshū many times over the course of the period . In the end , the downfall of the Shōni , marked by Shōni Fuyuhisa ' s 1559 defeat at the hands of Ryūzōji Takanobu , brought an end to the Sō clan ' s territorial aspirations on Kyūshū . Following a period of increased " wakō " predation , the Joseon - Japanese " Treaty of Tenbun " in 1547 ( " Tenbun 11 " ) limited trading to the Joseon port of Pusan and also limited Sō clan commerce to 20 ships annually . The Sō clan submitted itself to Toyotomi Hideyoshi in 1587 and supplied troops for the Japanese invasions of Korea ( 1592 - 1598 ) . Soon afterwards , however , the clan took it upon themselves to repair relations with Korea , sending a number of envoys under the guise of being official shogunal envoys . In doing this , the clan sought to improve their position with both the shogunate and Korea . There is evidence that they altered official documents from both to make themselves look better to the other The Sō did not participate in the Battle of Sekigahara in 1600 . The " tozama " Sō clan were allowed to continue to rule Tsushima . Tsushima and the area around Tashiro in Hizen Province were included in the clan ' s fief ( " han " ) , rated as worth 100 , 000 " koku " for its diplomatic and economic importance , though its agricultural production was actually under 30 , 000 " koku " per year . After the clan managed to resume diplomatic relations with Korea , the Tokugawa shogunate entrusted it with diplomatic negotiations and trade with Joseon . After the abolition of the han system in 1871 , the last known head of the clan , [ some say the family has gone secret ] Sō Shigemasa ( Yoshiaki ) , was made Governor of Izuhara Prefecture ( Tsushima , renamed ) .