On April 13, 1592, the Imjin War broke out just as Yi had foreseen. Hundreds of thousands of Japanese troops attacked Korea while it was not prepared for war. Yi analyzed the war situation thoroughly upon the outbreak of the war, and went to battle on May 4, 1592, with full preparation. He defeated the Japanese forces by destroying 42 Japanese ships in the naval battles of Okpo, Happo and Jeokjinpo. He continued to have successive victories in the battles of Dangpo and Danghangpo in June, the battles of Hansando and Angolpo in July and throughout the battles of Busan in September. The overwhelming victories of Yi’s naval fleet boosted the morale of the Korean Navy, and led to Korean control of the south coast of Korea. Yi outmaneuvered the Japanese forces that had previously advanced northward through Seoul and Pyongyang attacking from the sea and land.
On August 15, 1593, Yi was appointed to Commander-in-Chief of the Naval Force of the South (Samdo Sugun Tongjesa in Korean) in recognition of his outstanding wartime service. By this time, Japanese forces were stuck in a quagmire due to the successive victories of the Korean Navy, the appearance of Korean militias (Uibyeong in Korean), and the intervention of the Chinese Army. Japan had no choice but to initiate tedious peace talks with Ming China that lasted for 45 months. The peace talks did not include Korea because of Korean opposition to peace negotiations with the invading forces of Japan.
During this period, Yi continued his best efforts to increase the fighting power of the Korean Navy by recruiting and training soldiers, building arms and battleships, reserving gunpowder, and securing provisions. At the same time, he conducted naval operations under the unfavorable conditions of infectious disease and a shortage of provisions. According to his war diary entries for March of 1594, he himself suffered from a disease. Yi commanded the war despite his ailing body. Additionally, he was able to successfully procure large amounts of food for the war by managing land called Dunjeon, fishing, and producing salt, which proved his notable management ability.
Unfortunately, despite all of his efforts and accomplishments, he became entangled in domestic political strife and was eventually deprived of his rank as Commander-in-Chief and escorted to Seoul as a criminal in February of 1597. He faced the threat of the death penalty after suffering from brutal torture, but was eventually released from prison thanks to the efforts of many people to spare his life. However, he was forced to serve in war as a commoner, which is referred to as the punishment of ‘Baegui Jonggun’ in Korean. What was worse, he lost his beloved mother at this time. However, the General who took the place of Yi as Commander-in-Chief, General Won Gyun, was utterly defeated and died at the battle of Chilcheollyang in July of 1597.
Immediately upon the news of the disastrous defeat, Yi undertook a patrol through Korean coastal areas with his subordinates for a month in order to find solutions to recover the Korean Navy. He also reorganized the scattered soldiers and gathered weapons and supplies. On August 3, 1597, he was brought back to lead the navy upon receiving an official warrant of reappointment as Commander-in-Chief of the Naval Forces of the South. By this time there were only 13 Panokseon battleships left under his command. With this small fleet, Yi faced 133 enemy ships at the battle of Myeongnyang. Despite being overwhelmingly outnumbered, he triumphed over the Japanese with his distinguished leadership, outstanding tactics, and knowledge of the geographical features of the region. This victory stopped Japan’s attempt to advance into the west sea that was spurred by its victory at the battle of Chilcheollyang. Sadly, Yi lost his third son due to his victory at Myeongnyang because the defeated Japanese forces raided his home in Asan and retaliated against his family. Afterwards, Yi concentrated all his efforts on rebuilding the navy, while moving his naval base to Gohado, and later to Gogeumdo Island. Jingbirok, a chronicle of the Imjin War by Yu Seong-ryong, recorded that Yi moved his base to Gogeumdo on February 17, 1598, with 8,000 soldiers and 53 battleships.
When the Japanese warlord Toyotomi Hideyoshi died on August 18, 1598, Japanese forces rushed to withdraw from Korea. A combined Korean-Chinese fleet chased after the retreating Japanese army and had the last battle of the war off Noryang in November of 1598. Yi was killed by enemy fire during this battle. Mortally wounded and dying, he asked that no one be told about his death, concerned about its impact on the morale of his troops. The battle of Noryang was one of the biggest triumphs and most decisive battles that proved the patriotism and supremacy of the Korean Navy. Throughout the Imjin War, Admiral Yi Sun-sin was a preeminent warrior who was never defeated in at least 23 battles under his command for seven years. However, he was not just a brilliant commander, he was also a true leader who wholeheartedly loved his country and its people, and sacrificed himself to protect them. Hence, Koreans regard him as one of the greatest heroes of Korean history.