Yi Suil (; 1st day, 4th month of 1554 – 27th day, 5th month of 1632) was a Korean general of the mid-Joseon period. Born in Chungju in 1554, Yi Suil passed the special military examination in 1583 and began his service on the northern frontier.
In 1592, while serving as Magistrate of Changgi, Yi Suil faced the outbreak of the Imjin War broke out. He marched with his forces toward Gyeongju but was defeated, after which he served under Han Hyosun in military actions at Yŏnghae, Andong, Yecheon, and elsewhere. In 1593, Yi Suil was appointed Provincial Naval Commander of Left Gyeongsang, established a naval base in Changgi, and reactivated operations of the Left Gyeongsang Naval Forces. As Naval Commander, he distinguished himself in engagements at Ulsan and Ch'ilp'o.
After the Imjin War, Yi Suil served as Provincial Military Commander of North Hamgyong, overseeing campaigns to pacify Not'o, Suŭlho, and Kyoro settlements. as well as construction projects, including the rebuilding of Jinju Fortress and the creation of a new fortress at Songjin. During the reign of King Gwanghaegun, he was repeatedly entrusted with command in the north, serving three separate terms as Provincial Military Commander of North Hamgyong.
During the reign of King Injo, Yi Suil played a role in suppressing Yi Kwal's rebellion in 1624, securing victory at the Battle of Anhyŏn. For his service, he was credited as a Second-rank Chinmu Merit Subject. In subsequent years he held several high offices, including Commander-in-Chief of the Naval Forces of the South, Minister of Justice, and Commander of the Defense of Namhansanseong. In the 3rd month of 1632, at the age of seventy-nine, Yi Suil petitioned for retirement, which King Injo approved. Yi Suil died on the 27th day, 5th month of 1632 at his residence.
Early life and career Yi Suil was born on the 1st day, 4th month of 1554 at Sigok () in Chungju, Joseon. He belonged to the ; his father, Yi Ran (), was posthumously invested with the title Great Lord of Wŏlsŏng, while his mother was a daughter of U Tamnyŏng () of the Danyang U clan. Following his father's death in 1582, Yi, together with his elder brothers Yi Yuil () and Yi Kŭgil (), tended to their mother while preparing for the state examinations.
In the 1st month of 1583, when instigated a major uprising in Kyongwon, the court announced a special military examination in the 5th month, with the final test held in the 8th month. To reinforce defenses along the northern frontier, the quota of successful candidates was significantly increased, resulting in the appointment of 500 military officials. Yi Suil passed the examination, receiving a thirdclass placement with a ranking of 375th.
In 1584, Yi Suil entered service as a Probationary Record Keeper () at the Office of Military Training (). Two years later, in 1586, he attained the post of Junior Ninth-rank Military Officer () at Sonongbo () in Samsu, Hamgyong Province, after which he served under the regional commander Sin Kak. In 1590, he was appointed Royal Messenger (), and the following year, in 1591, he assumed the magistracy of .
Imjin War
Outbreak of the Imjin War On the 14th day, 4th month of 1592, following the outbreak of the Imjin War, Yi Suil, acting in accordance with the provisions of the , a regional defense strategy, mobilized his forces toward Gyeongju. Together with Pak Ŭijang, then serving as Executive Assistant to Magistrate () of Gyeongju, Yi committed himself to the defense of Gyeongju Fortress, implementing a siege posture by sealing the city gates. However, on the 21st day of the same month, the Japanese Second Division under Katō Kiyomasa advanced on the fortress. Faced with mass desertions and an overwhelming numerical disadvantage, the defenders were routed.
Yi consequently withdrew through the western gate of the fortress and retreated to Changgi. On the 23rd day, 4th month of 1592, when a Japanese detachment pressed forward into Changgi territory, Yi positioned his forces outside the local fortress but was once more compelled to retreat in the face of superior enemy strength. Subsequently, he joined , Magistrate of , to conduct coordinated operations against Japanese incursions. For his contributions in repelling enemy forces that had entered Yŏnghae, Yi was received promotion in the 6th month to the senior third rank of Grand Master for Comprehensive Governance (). He also participated in the defense of under Han Hyosun, who, by this time, had been elevated to Provincial Governor () of Left Gyeongsang.
Yi was appointed Magistrate of Miryang in the 10th month of 1592. Yet, in the following month, the Office of the Inspector-General filed a petition contending that his recent promotion had been excessive, as his military record lacked exceptional distinction. Consequently, his rank was demoted to the junior fourth rank of Grand Master for Court Dissemination (), though he retained the magistracy of Miryang. While in office, Yi participated actively in military campaigns across Andong, Yecheon, and surrounding districts under Han Hyosun's command. In recognition of his performance, he was appointed on the 10th day, 3rd month of 1593 to the position of Provincial Naval Commander () of Left Gyeongsang.
Provincial Naval Commander of Left Gyeongsang With the outbreak of the Imjin War, Gyeongsang Province—serving as the principal corridor of Japanese advance—was plunged into temporary disorder, while the Left Gyeongsang Naval Forces were virtually incapacitated, conducting little to no effective operations. Consequently, when Yi Suil assumed the office of Provincial Naval Commander of Left Gyeongsang, his initial activities were conducted primarily on land rather than at sea. On the 8th day, 6th month of1593, in concert with Kwŏn Ŭngsu, Provincial Military Commander of Left Gyeongsang, Yi launched an assault upon Japanese positions at Ulsan, during which their encampments were set ablaze, more than fifty were slain, and many others sustained injuries.
Drawing upon his earlier tenure as Magistrate of Changgi, Yi established a naval base at P'oip'o (), the naval garrison of Changgi, where he revived the operations of the Left Gyeongsang Naval Forces. From this base, he convened his subordinate naval officers, requisitioned fishing vessels, and oversaw the construction of warships, thereby enlarging his force. In the 7th month of 1593, when seven Japanese ships advanced, Yi, together with Mun Kwando (), Commander of Ch'ilp'o, engaged them in combat and succeeded in capturing four enemy vessels. For this success, Yi was elevated to the junior second rank of Grand Master for Excellent Virtue ().
Although Yi's fleet achieved successes at sea, the limited size of his forces and the rough seas along the East Coast restricted operations, and the Left Gyeongsang Naval Forces frequently served in the capacity of land troops, as in the 11th month of 1593, when they participated in the defense of Gyeongju following the Battle of Angang. Meanwhile, in recognition of their contributions, the court, during the winter of 1593, dispatched Royal Messenger To Wŏnnyang () to provide military support, including the construction of nine warships placed under Yi's command. Building upon this reinforcement, Yi's forces captured scattered Japanese troops along the southern reaches of the East Coast. In the 2nd month of 1595, when Japanese warships intruded at Chikchŏndo () in , Yi, in alliance with Hong Ch'angse, Magistrate of Yŏnil, engaged and defeated them. Shortly thereafter, following the death of his mother, Yi briefly returned to his family home, but soon resumed his duties at headquarters.
After Provincial Naval Commander In 1596, Supreme Commander () sought to bring Yi Suil into his service as subordinate general. However, that autumn, as Nurhaci's power expanded, Yi was instead appointed Magistrate of Hoeryong to reinforce the northern defenses. Shortly thereafter, on the grounds that the southern provinces held greater strategic importance than the northern frontier, he was reassigned as Magistrate of Naju in the 1st month of 1597 . Yi, however, did not immediately assume this post. Remaining in Hoeryong, he was entrusted with the troops assembled by Chŏng Munbu, Magistrate of Kilju, and ordered to lead them southward. Before he could formally take office, he was instead appointed in the 4th month of 1597 as Magistrate of Seongju, where the Supreme Field Headquarters had been established.
In the 7th month of 1597, with the outbreak of the Chŏngyu War, the Japanese launched a renewed offensive into the Korean interior. In the 9th month, Yi, alongside Ko Ŏnbaek, Mun Sinŏn (), and others, engaged Japanese forces in the Seongju region, where he reportedly killed twenty-one enemy combatants. At the beginning of 1598, however, Yi fell ill and was unable to resume active duty. The Chŏngyu War ended in the 11th month of 1598 with the Japanese withdrawal, and in the following 12th month Yi was appointed Provincial Military Commander of Right Gyeongsang. Yet in the 1st month of 1599, in the aftermath of the withdrawal, Yi entered into conflict with the Ming official over postwar arrangements concerning Suncheon Castle. As a consequence, Yi was dismissed from office and subjected to interrogation. Following a reinvestigation, during which he was defended by the Ming general Mao Guoqi, Yi was reinstated on the 20th day, 2nd month of 1599 as Magistrate of Seongju.
Northern border conflicts
Siege of Not'o settlement Meanwhile, Nurhaci, who viewed the Pŏnho—long allied with Joseon—as potential targets of annexation, initiated attacks on the Not'o settlement along the upper Tumen River in the 1st month of 1598. Tensions further escalated in the 3rd month of 1599, when Jurchens of the upper Tumen ambushed Joseon reconnaissance forces, prompting the court to prepare a punitive expedition against the Not'o settlement. To this end, Yi Suil was appointed Provincial Military Commander of North Hamgyong in the 7th month of 1599.
In the 3rd month of 1600, following a Jurchen raid on Puryong, Joseon court seized upon the incident as grounds to implement the planned campaign against the Not'o settlement. On the 14th day, 4th month of 1600, Yi organized a force of three thousand troops into three divisions—Center, Left, and Right—dispatching them from Musan, Ŏyugan (), and Pungsan respectively, while he accompanied along the Center Division. At dawn on the 15th, the three divisions executed a coordinated offensive. Yi directed the Left Division to cut off the enemy's retreat, while advancing with the Center and Right Divisions. The Joseon forces destroyed seven settlements, including the Not'o stronghold, before withdrawing the following day to Pungsan Fort at Hoeryong.
Siege of Suŭlhŏ and Kyoro settlements In the 8th month of 1600, the Suŭlho and Kyoro settlements of Onsong, in alliance with the Hūlun of the Haixi Jurchens, launched attacks against rival settlements and assisted in the siege of Onsong. In response, Yi Ŭnghae (), Magistrate of Onsong, devised a punitive plan against the two settlements, with Yi Suil placed in command of the operation. On the 20th day, 12th month of 1600, the court authorized the campaign, and Yi Suil and Yi Ŭnghae scheduled it for the 1st month of the following year.
Yi Suil assembled troops primarily from the of Hamgyong Province, organizing them into three divisions—Center, Left, and Right. Command of these divisions was assigned respectively to Army Commander () Yi Yujik (), Magistrate of Onsong Yi Ŭnghae, and Magistrate of Yi Hongbo (). On 27th day, 1st month of 1601, Yi Suil ordered the simultaneous advance of all three divisions against the Suŭlho and Kyoro settlements. Through a surprise attack, the Joseon forces decisively destroyed both communities.
Fortifications