The Oei Invasion (応永の外寇)

The Oei Invasion (known in Korea as the Gihae Eastern Expedition, with Gihae being a reference to the year 1419) refers to a raid on the Japanese island of Tsushima by Joseon Korean forces that occurred in 1419 (the 26th year of the Oei era, from which the Japanese name is taken), meaning that it took place during the Muromachi period according to the standard classification of Japanese history. Because the major battle was fought at Nukadake on Tsushima, it is also referred to as the Nukadake War.

Transitions concerning the incident
Background
Around the fourteenth century, Wako (specifically, the early Wako), a multiethnic group of pirates that included Japanese, Chinese, and Koreans, began increasing their activities in the seas from northern Kyushu in Japan to those around the Korean peninsula and along the coast of China; in addition to the formal tributary trade missions, it was now also permissible for trade to be conducted between Japan and Korea by private merchants, adding a more international flavor to the maritime world of East Asia during this period.

During the days of the early Wako, the Korean peninsula was not only one of the Wako's principal targets, but also their base of operations, and because the Joseon dynasty had only just been established in Korea, the new government lacked the independent ability to preserve peace, making it extremely difficult for them to suppress Wako pirate activity. At this point, the Joseon dynasty wanted to cull society of its multiethnic character, which had been the norm during the Goryeo period (918 - 1392), and achieve a monopoly over trade profits by bringing trade under government control, so they requested that the Muromachi bakufu, the tandai (military commissioner) of Kyushu, and Sadashige SO, the de facto ruler of Tsushima island, bring both Wako activity and private trade under control.

However, Sadashige SO, who had tried hard to restrict Wako activity, fell gravely ill in September 1417 before eventually dying of this illness in April 1418, and when Sadamori SO succeeded Sadashige as ruler of Tsushima, the So clan's ability to police the seas around Tsushima was hampered and weakened by Sadamori's youth and inexperience; this meant that the Wako, who had been held in check by Sadashige, erupted into fresh activity near Tsushima, inflicting heavy damage on Japan and Korea.

Military action
With King Sejong the Great having ascended to the throne in Joseon Korea only the previous year (1418), former King Taejong still held de facto power, and it was Taejong who decided to launch an expedition against Tsushima in June 1419 under the pretext of pacifying the Wako, dispatching a fleet of 227 ships and 17,285 soldiers to invade Tsushima, with general YI Jong Mu in command. At this point in time, it was known by Korean military planners that the most powerful inhabitants of Tsushima were away on voyages to Ming China and elsewhere, and moreover as the invasion was launched they ordered the simultaneous internment of all Japanese merchants living and operating within Korea, revealing how carefully they had planned this assault to take advantage of the weakness of Tsushima's defensive preparations and target the general population (not only the Wako). Moreover, Taejong included the unfounded claim that 'according to ancient texts, Tsushima is historically a vassal state of Gyeongsang province' in his order to attack. On June 17, the invasion fleet set sail from Geoje island, but the headwind soon drove them back, leading to a second departure on June 19. At around noon on June 20, the Korean army landed in the vicinity of Osaki inlet (according to the True Record of the Joseon Dynasty, it was at Mamejiru inlet). Of the ordinary ships in this area, they burned 129 and captured 20, and burned down 1939 dwellings as well as killed 104 inhabitants of Tsushima (or according to the True Record, they took a total of 114 heads). But around June 26, Korean forces fell into an ambush by warriors from Tsushima in the Nii district (or the Amaro district according to the True Record), in which they suffered significant casualties, prompting Jong Mu YI to withdraw his army back to Osaki inlet and leaving the outcome of the war an uneasy stalemate. On June 29, an envoy from the Korean forces was sent to the So clan demanding among other things that they agree to convert Tsushima into a vassal state of Korea, but the So clan refused.

The military situation had reached a stalemate thanks to the counterattack by Tsushima forces, and the escalated casualties among the Korean army prompted them to accept the peace proposal sent by the Tsushima forces and carry out a complete withdrawal back to Geoje Island on July 3. Korean casualties exceeded 2500 dead or wounded according to Japanese sources, while one entry in the Record of Seojong's Reign lists 100 and some dozens more killed, and another later entry lists 180 killed. But as this would amount to only about 1% of their total force, it is difficult to imagine why the Korean army would have made peace, leading some to suspect that actual casualties were much higher, a possibility reinforced by the Korean's own records clearly indicate that this invasion was a defeat. According to the entry in the True Record of the Joseon Dynasty for the fifth day of the eighth month, the Japanese forces suffered 20 dead in the fighting, while the Korean army had suffered over 100, demonstrating that in battles between the two countries' regular armies, the Korean forces were no match for the Japanese. The weakness of the Korean army was further attested to in an entry discussing Chinese captives that they had rescued from Tsushima which said 'Having seen in detail the weakness of our army in Tsushima, we cannot allow our captives to return to China,' as well as by the fact that Sil PAK was held responsible for the defeat and imprisoned; it may be surmised that only the influence of the Korean people enabled Jong Mu YI to escape responsibility.

Furthermore, a report reached the Koreans on July 3 saying that several dozen Wako ships from China were active in the sea around Hwanghae province (located in Korea) and had begun rampaging along the coast, after which a discussion ensued as to whether this should be used as a pretext for a second punitive expedition against Tsushima, but no such invasion was ever launched.

The following year a peace treaty was concluded between Japan and Korea, and Hui Gyeong SONG (also known as Nosongdang) was sent as special envoy.

Due to this incident, the various daimyo of Tsushima and northern Kyushu became even more repressive, and as a result of conciliatory measures intended to win the allegiance of the pirates (such as the offer to naturalize them as Japanese citizens), the power of the early Wako faded away. When news of the Korean attack on Tsushima reached mainland Japan, speculation ran rife that this was the second advent of the earlier Mongol invasions. Consequently, the Muromachi bakufu (Japanese feudal government headed by a shogun) sent envoys to Korea to conduct an investigation into whether the rumors of a new invasion from mainland China were accurate, and had the envoys endeavor to get to the bottom of the matter.

In both the 'Ying huan zhi lue' by the Qing-period Chinese writer Jiyu XI and the 'Tongsa kangmok' by the Joseon-era Korean writer Chong Bok AN, the motivation of the Wako is considered to be retribution for Goryeo-era Korea's participation in the two invasions (by the Mongols) of Japan, and as such some have argued that the actions of the early Wako, who appeared before the Oei Invasion, should be considered a local effort to recapture lost wealth and a war of vengeance against Korea and China, so these pirates should not be called Wako (Japanese pirates); according to this theory, only pirates appearing after this point should be considered 'Wako,' or Japanese pirates.

Concerning claims by Koreans of territorial sovereignty over Tsushima

On June 17, 2005, in the city of Masan in South Gyeongsang province in the Republic of South Korea, the date of June 19 of every year was declared "Tsushima Day" by the local government, with more than 400 citizens of Masan participating in "the proclamation ceremony." It was such a sudden announcement that both the Japanese and South Korean governments expressed surprise, and this act was viewed by some in Japan as retaliation for the establishment of 'Takeshima Day' in Shimane Prefecture, which happened just before the Korean proclamation. However, Japan did not escalate matters, so the South Korean government sought to have the decision made by the city of Masan repealed. Note that June 19 is the day "the punitive force" set sail from Masan inlet.

Some Koreans advance the claim that Tsushima was a vassal state of Korea during the Joseon dynasty and should therefore be under South Korea's territorial control; proponents of this view cite 'the military expedition to suppress the Wako' that was sent against Tsushima during the Oei Invasion as proof of this. However, the sole purpose of the expeditionary force's invasion was to subjugate the Wako pirates, and Joseon Korea's demand that Tsushima become a vassal state was only an attempt to reassert Korea's titular control over Tsushima as an "outer domain" in order to encourage them to bring the Wako under control; it is difficult to find evidence of any suggestion that the Koreans were planning to make Tsushima a permanent vassal state. Moreover, the Korean army withdrew after their invasion ended in a stalemate, and the focus of the agreement reached in subsequent negotiations and its execution was once again the suppression of the Wako, making this expedition an extremely weak justification for asserting that Tsushima had been a vassal state of Korea. No official interpretation has been forthcoming from the city government of Masan that adequately resolves this discrepancy between the historical facts and the grounds for their claims that Tsushima is part of Korea. On September 17, 2006, three volumes analyzing military strategy as far back as Joseon-era Korea were published by a research group that compiles Korean military annals, one of which (a "compilation of Joseon-era military strategy") exmines the Oei Invasion. According to this volume, militarily "the expedition" was a failure, but did win them the following document of capitulation: 'Tsushima's land is barren, making it difficult to eke out a living there. If you would allow us islanders to live on Geoje Island and its neighboring islands, and honor us by viewing Tsushima as being under Korea's dominion and granting us a new provincial label and seal, we, as your grateful retainers, will obey your orders accordingly." As such, the analysis concludes that the expedition was a success politically and diplomatically. The analysis further concluded that in granting Tsushima's So clan a seal that bore the inscription "So clan of Yugawara City" in July 1420, Joseon Korea failed to gain recognition for their sovereignty over Tsushima; if at that time Korea had refused to recognize Tsushima's autonomy, and instead sent an official to put Tsushima's government under their direct control, Tsushima would almost certainly be under Korea's territorial control today (as reported in a Rengo news broadcast of September 17, 2006).

The True Record of King Sejong's Reign expressly states that the invasion ended in military defeat, and it also contains an entry for the tenth day of the intercalary first month of 1420 that says, "What Shiokatsu had said was indeed spoken;" yet no mention is made of the content of the above-mentioned document of capitulation. Moreover, although he claimed to be an emissary of Korea, Shiokatsu (also known as Keido SHIN) is believed to have been an imposter, and in the entry for the seventh day of the fourth month of 1420, Anri KYU, the official envoy of Sejong, is recorded as having been highly critical of Shiokatsu, saying: "What Shiokatsu has said is rash and thoughtless. To attack Tsushima is to attack Japan." Furthermore, Hui Gyeong SONG, the special envoy dispatched to Japan from Korea, tried to vindicate Korea's invasion of Tsushima by saying "We believed our actions would please the King of Japan," which resulted in the incident being considered a diplomatic failure as well. But the analysis conducted by the research center that compiles Korean military annals ignores the entry in the True Record of King Sejong's Reign that describes the expedition as a military defeat, focusing exclusively on the words of a man who claimed to be an emissary, leading to doubts about whether they intentionally distorted historical records to support their explanation. Also, on July 13, 2007, Sung Man KIM, a former high-ranking officer specializing in naval tactics (specifically, a vice-admiral in the Navy reserve) spoke out on the Korean National Security Net against the claim of sovereignty over Takeshima island (by Shimane Prefecture) made in Japan's Self-Defense white paper, and also requested that he be allowed to draw up a plan for the invasion of Tsushima. In 2008, 50 members of the South Korean National Diet submitted a resolution calling for a referendum over whether Tsushima should be returned to Korean sovereignty, and 50% of South Korean people supported the proposal.