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June 1419 (Ōei 26)
Tsushima Province (present-day Tsushima City, Nagasaki Prefecture)
Forces
Joseon Korean Forces
Tsushima Sō Clan Forces
About 18 min read
The Oei Incursion (Oei no Gaikō) was an incident that occurred in June of the 26th year of the Oei era (1419), during the Muromachi period, when the Yi Dynasty of Joseon invaded Tsushima with a large military force. The Japanese side called this the 'Oei Incursion,' while the Korean side referred to it as the 'Gihae Eastern Expedition.' For the first time in 140 years since the Mongol Invasions (1274 and 1281), it was an organized foreign military invasion of Japan, dealing a major shock to both the Muromachi shogunate and the Sō clan of Tsushima. In this battle, several hundred Tsushima samurai engaged an army exceeding seventeen thousand, and through their fierce resistance created a stalemate. The Japanese swords wielded by the Tsushima samurai embodied the essence of bushidō in the face of overwhelming numerical disadvantage.
Throughout the Muromachi period, wokou (Japanese pirates) based in Kyushu, Tsushima, and Iki repeatedly raided the coasts of Korea and China. The wokou conducted plundering and kidnapping in various locations, inflicting severe damage on villages along the Korean coast. Yi Taejong (r. 1400–1418) had continued negotiations with Tsushima as a countermeasure to wokou, but in the 25th year of Oei (1418), Sō Sadashige, the guardian of Tsushima who had devoted himself to suppressing the wokou, died suddenly. His successor was the young Sō Sadamori, and the Korean side judged that if the new heir's control weakened, the wokou would become active again. The retired king Yi Taejong, who held actual power under the new king Yi Sejong (r. 1418–1450), decided on military subjugation and sent an unprecedented force of 227 warships and 17,285 soldiers to Tsushima.
On June 20 of the 26th year of Oei (1419), a Korean advance fleet appeared at Ozaki Bay at the northern tip of Tsushima. The main force then began landing and conducted searches, plundering, and arson throughout the island. They burned 1,939 houses, killed 114 islanders, and captured and removed 129 prisoners, inflicting severe damage on the island. However, the Korean forces encountered unexpected difficulties at the Battle of Nukadake on June 29. Though the Tsushima forces numbered only about 600 samurai, they deployed a surprise ambush strategy using mountainous terrain to their advantage. Under Sō Sadamori's command, the samurai lured the Korean forces into the interior, drew them into steep mountain passes, and surrounded and attacked them. In this battle, the Korean forces suffered hundreds of casualties, including four officers, either killed in combat or fallen from cliffs. Despite the overwhelming numerical advantage, the Tsushima samurai's fierce resistance considerably shook the morale of the Korean forces.
The Japanese swords used by the Tsushima samurai at this time were blades from the middle Muromachi period in the early 15th century. The Oei era represents a transitional period in which swordsmithing was shifting from mass production to refined manufacture following the intense warfare of the Northern and Southern Courts period. During the tumultuous Northern and Southern Courts period of the 14th century, many mass-produced blades called 'Oei Bizen' were forged to meet the frequency of battles, but with the 15th century, swordsmiths from Bizen Osafune, Yamato, and Yamashiro returned to refined techniques, and blade shapes also transitioned from the large tachi of the Northern and Southern Courts period back to more elegant, smaller tachi and uchigatana. Sō Sadamori's leadership of the Tsushima samurai in resistance against the Korean forces exemplified the bushidō of an elite minority, consonant with the cultural background of the Japanese sword: 'not relying on numbers, but deciding victory through individual martial skill and the advantage of terrain.'
After the Battle of Nukadake, as the situation reached stalemate, Sō Sadamori pursued a diplomatic solution. Sō Sadamori negotiated with Korean military envoys, warning that 'the season of storms approaches, and continued encampment would endanger your forces,' and proposed a ceasefire and restoration of friendly relations. The Korean forces accepted this proposal, and on July 3, the entire army withdrew from Tsushima. The invasion lasted approximately 13 days. Subsequent Japan-Korea diplomatic negotiations were difficult, but friendly relations were restored by the 30th year of Oei (1423), and the 'Kaichoku Treaty (Kigai Yakujō)' was concluded in the third year of Kaichoku (1443). Through this treaty, the Sō clan of Tsushima formally acquired the exclusive right to manage commerce and trade with Korea, and Tsushima established itself as an 'intermediary island' between Japan and Korea.
News of the Oei Incursion reached the Muromachi shogunate, and the fourth shogun, Ashikaga Yoshimochi, deliberated on a response. Organized military invasion by foreign forces had not occurred since the Mongol Invasions, and the shogunate ordered the guardians of Tsushima and various Kyushu provinces to heighten their vigilance. However, the Muromachi shogunate lacked both the military capacity and the will to mount an active counteroffensive against the Korean peninsula, and the situation was ultimately resolved through the diplomatic efforts of the Sō clan. This incident, while exposing the limits of the Muromachi shogunate's foreign military influence, demonstrated how local samurai clans (the Sō clan) could defend their territory through independent diplomatic and military action, and is thus positioned as an important event in Japanese medieval history. The valiant struggles of the Tsushima samurai were remembered in later generations as exemplifying the indomitable spirit of samurai who fight courageously despite their fewness in number, and as an expression of the spiritual armament symbolized by the Japanese sword.
The Oei era (1394–1428) was a crucial transitional period in the history of the Japanese sword. The intense warfare of the Northern and Southern Courts period (1336–1392) promoted mass production of blades, a time when large quantities of swords from Bizen Osafune, collectively called 'Oei Bizen,' were forged. Compared to the large tachi of the Northern and Southern Courts period, Oei Bizen blades became shorter and less curved, evolving into a practical form that balanced portability with combat effectiveness. The blade patterns (hamon) frequently featured gun-no-me (interlocking wavy patterns) and ko-midaré (small irregular patterns), reflecting the degree of completion as uniform mass-produced items rather than refined craftsmanship. It is surmised that the Tsushima samurai at the Oei Incursion wielded precisely these Oei Bizen blades or swords from the Yamato and Yamashiro schools. The historical process by which the Japanese sword's form evolved from combat experience and fused the spirit of the age's samurai with practical function was refined through verification in actual battlefields such as the Oei Incursion. The Oei Incursion, as a symbolic event of an age when the Japanese sword functioned not merely as an art object but as a practical combat weapon, holds profound significance in the history of swordsmithing.
The Oei Incursion is frequently compared with the Mongol Invasions of the Kamakura period (the Bunei Campaign of 1274 and the Kōan Campaign of 1281). At the Mongol Invasions, Kamakura samurai faced the alien tactics of Mongol formations, poisoned arrows, and tetsuhau (explosives), and were forced to abandon the conventions of one-on-one honor duels that characterized Japanese bushidō. The experience of the Mongol Invasions is thought to have influenced the practical evolution of the Japanese sword's form. By contrast, in the Oei Incursion, Tsushima samurai countered the large Korean army through mountain guerrilla tactics that maximized their geographic advantage. The Japanese sword is a weapon suited to close combat in narrow mountain spaces, and demonstrates its power in situations where a small elite of samurai demonstrates individual martial skill rather than massed formations. The battles of the Oei Incursion thus demonstrated that the Japanese sword, as a weapon embodying 'individual martial skill over collective power,' exerts its maximum force within the geographic and cultural context of Japan. The memory of this incursion was inscribed in the samurai culture of later generations in Kyushu and Tsushima, becoming a spiritual legacy that cultivated vigilance against external threats and fostered samurai pride.
The Sō clan of Tsushima, based on the island since the Kamakura period, developed a unique samurai culture through their mediation of Japan-Korea trade. The Sō clan's retainers were skilled samurai well-versed in Tsushima's harsh mountainous terrain and intricate coastline, and the swords they carried were largely procured from swordsmiths of Kyushu (particularly those of Higo and Ōsumi) or from Bizen Osafune. Due to Tsushima's scarcity of metal resources, the Sō clan channeled wealth gained from trade with Korea and China toward the purchase of swords and armor, thereby maintaining their samurai force's fighting strength. The 600 samurai led by Sō Sadamori in the Oei Incursion exemplified the essence of Tsushima's martial culture. The battles of the Oei Incursion, in which a small elite courageously resisted an overwhelming foreign army through superior terrain, surprise tactics, and spiritual advantage, remain celebrated as a concrete realization of the ideal of Japanese bushidō—the fusion of individual martial skill and collective maneuver in samurai bearing the Japanese sword. The descendants of the Sō clan continued to serve as daimyo of Tsushima into the Edo period, managing Japan-Korea diplomacy, and Tsushima's martial culture was transmitted down through the Meiji Restoration.