[11] The immediate cause of the conquest was the incident on May 5, 1419, when 39 Japanese warships captured Cape Dodu (都豆音串) in Biin County (庇仁縣) on their way to Ming dynasty China.
The conflict between the court in Nara and Kyoto caused the central authority to be unable to limit pirate activity, while indirectly exacerbating it due to the accompanying political chaos.
[1] The Goryeo military was busy with an expedition into the Liaodong in 1388, which caused the defenses in the south to be neglected and the Wokou to use this opportunity to raid the island of Yanggwang (楊廣道).
Kim Sa-hyeong (김사형, 金士衡) Namjae (南在) and Yi Mu (李茂) were tasked with the suppression of Tsushima and Iki island and returned to Seoul in January 1397.
[1] Following January 1398 Joha (朝賀) (On the occasion of the winter solstice where vassals and courtiers would pay their respects to the king) envoys from Tsushima came to Joseon court almost every year and would bring offerings and receive rice and beans in return.
The Joseon government subsequently announced that traders are allowed entry to Busanpo (釜山浦) and Naeipo only on ships with a certificate of passage.
Afterwards, there were many Wa (倭, ethnonym for Japanese) living in this area which according to the government officials caused unrest, while also attempting to sabotage the state authorities.
In response in March 1418 Taejong he set up Japanese enclaves in Yeompo (鹽浦) and Gabae-ryang (加背梁) in Gyeongsang province.
[1] Joseon subsequently asked the Ashikaga shogunate and its deputy in Kyūshū to suppress pirate activity, favoring legitimate traders.
In exchange for certain privileges, it gave authority to Sō Sadashige, the de facto ruler of Tsushima Province, over ships sailing from Japan to Korea.
On June 9, 1419, Taejong declared a war against Tsushima, citing that it belonged to Joseon, and Yi Jongmu was chosen to conduct the expedition.
[7] On the other hand, according to Kanmon Nikki (看聞日記), a diary written by Prince Fushiminomiya Sadafusa (ja) on August 13, citing a report sent by Shoni Mitsusada (ja), the Tsushima side entered the battle with a cavalry force of about 700 cavalrymen and fought especially hard all day on June 26, killing over 3,700 Koreans by July 2.
[23] In exchange, the Sō clan provided copper, tin, sulfur, medicinal herbs, and spices from both Tsushima island and Japan.
In a letter to Sō Sadamori issued on 15 July, the Joseon claimed that Tsushima belonged to Gyeongsang Province and asked him to leave the island, either by coming to the Korean mainland and submitting himself or by retreating to Japan.
Sejong Sillok, the same self-claimed envoy, agreed to Korea's proposal to put Tsushima under the rule of Gyeongsang Province.
[25] The diplomatic mission left the Joseon capital in early 1420 and met with Soda Saemontaro on Tsushima as Sō Sadamori stayed with the Shōni clan in Hizen Province.
Having accomplished his mission, he left Kyoto in June, returning to Korea after completing negotiations with the Shōni and Sō clans in Kyūshū.
In April 1421, a letter from Sō Sadamori demanded the return of Japanese captives and pointed out Korea's claim over Tsushima.
It is noted that the Japanese envoy took advantage of the shogunate's authority, which can frequently be found in the Sō clan's later diplomatic talks with Korea.
Korea repeated necessary involvement in the island's operations, by aiding in the recent famine and general trade route policing led the ruler to declare the land would be reclaimed by force in order to protect the integrity of the region.
In January 1420, a Japanese envoy visiting Seoul requested to have a copy of Tripitaka Koreana, a comprehensive Buddhist script held in great regard as a Korean national treasure.