Ōei Invasion

Background

From about 1400, despite its incorporation into the Japanese political order (this incorporation was however limited, to the point that Japanese authorities, regional and national, were unable throughout most of Japanese history to control and limit pirate activity originating in this area) before the Goryeo, Tsushima were located on the front lines that defended Japanese territory for much of its history. Historically, a large part of Tsushima's economy was sustained by trade with Korea; it was used as a "frontier territory" and a diplomatic meeting place between Korea and Japan, but was considered historically by Koreans as a vassal or dependent state of Korea, and despite a variety of changes in terminology over the ages designed to indicate its status as being in the Japanese sphere of influence, it was considered by many Koreans to be Korean land under foreign occupation.

From the end of the Goryeo through the early Joseon, the coastal regions of Korea, their populations, and their resources were often the objective of Wokou raids.

In 1389, General Pak Wi (박위, 朴威) of Goryeo cleared the island of Wokou pirates, he burnt 300 ships and rescued more than 100 Korean captives. The Joseon Dynasty ordered a strengthening of Korean naval defenses, a strategic response to the constant threat posed by the pirates. In 1396, Korean official Kim Sa-hyeong (김사형, 金士衡) led a campaign into Tsushima.

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. When Sō Sadashige died in 1418, power was seized from Sadashige's infant son Sadamori (Tsutsukumaru) by Soda Saemontaro, a powerful pirate leader. Suffering from famine, pirates on Tsushima invaded Ming China in 1419. On the way to China, they invaded Korea's Bi-in and Haeju country after their requests for food were rejected.

After receiving reports of these incidents, the Korean court approved an expedition of Tsushima. Taejong, who had abdicated his throne in 1418 but was still a military adviser of Sejong the Great, favored a more offensive approach. On June 9, 1419, Taejong declared a war against Tsushima, citing that it belonged to Joseon, and Yi Jong-mu was chosen to conduct the expedition.

Invasion

The Koreans waited until a large Japanese fleet had left the island for a raid. Yi Jong-mu's fleet of 227 ships and 17,285 soldiers set off from Geoje Island toward Tsushima on June 19, 1419. The following day the fleet landed in Asō Bay (浅茅湾).

General Yi Jong-mu first sent captured Japanese pirates as emissaries to ask for surrender. When he received no reply, he sent out expeditionary forces, and the soldiers proceeded to raid the islanders and pirates and plunder pirate settlements. He found and rescued 131 Chinese captives of the pirates and 21 slaves on the island, burned 129 ships and 1,939 houses, and killed or captured 135 pirates.

On the 26th day, the Korean army was ambushed on land by a Japanese army in an ambush at Nii, and suffered 150 casualties. The ambush was known to the locals as the Battle of Nukadake (糠岳の戦い).

In the weeks that followed, a truce was negotiated with the Sō clan on the island. The Korean expeditionary force withdrew and sailed back to the Korean Peninsula on July 3, 1419. and Korea gave up occupation of Tsushima. In subsequent diplomatic exchanges, Tsushima would be granted trading priveldges with Joseon, in exchange for maintaining control and order of pirate threats originating from the island.

Aftermath

A treaty was negotiated between Joseon and the Sō clan on January 1420. The Joseon government agreed to grant the Sō clan limited trading privileges and access to three coastal Korean ports, under the condition that the Sō clan control and stop any coastal pirate raids. On the 6th day of the 7th month in 1422, an emissary from the Sō clan requested freedom for Japanese prisoners of war captured by the Korean forces. He offered tributes of copper and sulfur in return. On the 20th day 12th month in 1422, all Japanese prisoners of war were released.

In 1443, the Daimyo 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.

In 1510, Japanese traders initiated an uprising against Joseon's stricter policies on Japanese traders from Tsushima and Iki coming to Busan, Ulsan and Jinhae to trade. The So Clan supported the uprising, but it was eventually suppressed by local authorities. The uprising was later came to be known as the "Japanese riots in Southeast Korea" (삼포왜란, 三浦倭亂).

A more restrictive treaty was re-imposed under the direction of King Jungjong in 1512, but this Treaty of Imsin was enacted only under strictly limited terms, and only twenty-five ships were allowed to visit Joseon annually until "Japanese riots in Saryangjin" (사량진왜변, 蛇梁鎭倭變) in 1544.

The Korean export included rice, cotton, ginseng, and Sutra texts. In exchange, the Sō clan provided copper, tin, sulfur, medicinal herbs, and spices from both Tsushima island and Japan. The Sō clan became a trading hub between Korea and Japan and benefited greatly from it.

The relationship between Korea and residents of Tsushima Island greatly improved thereafter; there were numerous records of hospitality towards shipwrecked Korean sailors who ended up on the island, and merchants from Tsushima Island enjoyed special privileges in Korean ports.