Battle of Hansan Island

Admiral Yi Sun Shin, along with the small fleet of seven ships of Admiral Won Gyun, had fought two campaigns across the southern coast of Korea. For the third campaign, Yi Eok-gi joined with Yi Sun-sin at Yeosu on August 10 before rendezvousing with Won Gyun at Noryang on August 12. In all, the Koreans sank over 60 Japanese ships. Admiral Yi and the combined Korean fleet did not lose any ships and suffered only 11 killed and 26 wounded up to this point.

Given the importance that the Japanese navy had in supplying the army as it advanced along the Korean peninsula and prepared to invade China, Toyotomi Hideyoshi made it absolutely imperative to his commanders that the naval situation must be brought under control, the Korean fleets destroyed and the supply routes through the Yellow Sea secured. The Japanese commander, Wakizaka Yasuharu, was ordered to wait and combine his fleet with the forces of Katō Yoshiaki and Kuki Yoshitaka to seek out and destroy the Korean fleet. However, it would have taken some time for Katō and Kuki to assemble their ships, so Wakizaka went out alone with 73 ships. Wakizaka's fleet probably had the best warships fielded by the Japanese up to that point in the war. Out of the 73 ships, 36 were the large multi-decked atakebune, 24 the medium sized seki bune and 13 small kobaya scout ships.

In the meantime, Admiral Yi was planning a third campaign and worked with Admirals Won and Yi Eok Ki in combined operations and practiced arranging their fleets in a "crane's wing" battle formation. The formation was often used on land, but not normally used at sea. The combined fleets had a total of 54 panokseons and 2 or 3 turtle ships.

Admiral Yi's victory at Hansan Island effectively ended Hideyoshi's dreams of conquering Ming China, which was his original goal in invading Korea. The supply routes through the Yellow Sea had to be open in order for his troops to have enough supplies and reinforcements to invade China. Thus, Konishi Yukinaga, the commander of the contingent of troops in Pyongyang could not move further north due to lack of supplies, nor could more troops be sent to him because there was not enough food to feed them. It took five times the resources in food and men to move supplies via the land route over Korea's primitive roads. Furthermore, moving supplies overland left them vulnerable to attacks by regular Chinese and Korean forces as well as Korean irregular or guerrilla forces (the Righteous Armies, 의병, 義兵) that were becoming increasingly active as the war progressed.

After the battle of Hansan Island (and the Battle of Angolpo shortly afterwards), Hideyoshi found it necessary to give a direct order to his naval commanders to cease all unnecessary naval operations and limit activity to the immediate area around Pusan Harbor. He told his commanders that he would come to Korea personally to lead the naval forces himself, but Hideyoshi was never able to carry through on this as his health was deteriorating rapidly.

The battle of Hansan Island was the most important battle of the Imjin War. It ensured that all the fighting would be in Korea, not China, and that Pyongyang would be the furthest northwestern advance of the Japanese armies (to be sure, Katō Kiyomasa's second contingent's brief march into Manchuria was Japan's northernmost advance, however, Manchuria was not a part of Imperial China in the 16th century). It can be argued that the battle was one of the most important in Korean history up to that point. While Hideyoshi was unlikely to be able to invade China and conquer a large part of it, the battle of Hansan Island checked his supply routes and hindered his movements in Korea. Hideyoshi's larger war plans, supported in much written documentation, was nearly identical to Imperial Japan's blueprint for conquest in the second half of the 20th century.

George Alexander Ballard (1862–1948), a vice admiral of British Royal Navy, complimented Admiral Yi's winning streaks by the Battle of Hansando highly:

"This was the great Korean admiral's crowning exploit. In the short space of six weeks [actually about 9 weeks, May 7, 1592 – July 7, 1592] he had achieved a series of successes unsurpassed in the whole annals of maritime war, destroying the enemy's battle fleets, cutting his lines of communication, sweeping up his convoys, imperilling the situation of his victorious armies in the field, and bringing his ambitious schemes to utter ruin. Not even Nelson, Blake, or Jean Bart could have done more than this scarcely known representative of a small and cruelly oppressed nation; and it is to be regretted that his memory lingers nowhere outside his native land, for no impartial judge could deny him the right to be accounted among the born leaders of men."[5]

• Hawley, Samuel 2005 The Imjin War: Japan's Sixteenth-Century Invasion of Korea and Attempt to Conquer China. Republic of Korea and U.S.A.: Co-Published by The Royal Asiatic Society and The Institute of East Asian Studies, University of California, Berkeley.
• Turnbull, Stephen 2002 Samurai Invasion: Japan's Korean War. Great Britain: Cassell & Co.
• Sohn, Pow Key (edited by) 1977 Nanjung Ilgi: War Diary of Admiral Yi Sun-Shin. Republic of Korea: Yonsei University Press.

• The Four Campaigns of Admiral Yi during the Imjin Year (1592)
• Siege of Jinju (1592)
• Battle of Haengju
• Battle of Myeongnyang