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 Daimyō of Tsushima, Sō Sadamori 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.Tsushima tourist Association WEB site "1443　嘉吉条約（発亥約定）- 李氏朝鮮と通交条約である嘉吉条約を結び、歳遣船の定数を定める。これにより、宗家が朝鮮貿易の独占的な地位を占めるようになる。" 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.
