Excerpts from Books and Wikipedia
"In July of 1592, the Japanese commander who stood overlooking the Bay of Sacheon in Korea was probably quite pleased. The invasion of Korea was proceeding as planned; a force of more than 160,000 Japanese samurai had captured Pusan, Korea’s main port, then marched north and seized the capital city of Seoul. The great Japanese warlord Toyotomi Hideyoshi, the mastermind behind the invasion, was well on his way to extending his dominion to include Korea and, using that foothold, to conquer China. The only blotch on the invasion’s record of triumphs occurred when an unusually aggressive Korean admiral, Yi Sun-shin had caught and destroyed several dozen Japanese vessels. Now, this same Yi Sun-shin had been sighted sailing into Sacheon Bay. . . . The battle marked the combat debut of the turtle ship. Over the next decade, Yi and his turtle ships would play a key role in defeating Japan’s invasions of Korea, and Yi’s actions would establish him as one of the greatest admirals of all time." [Aldrete: Decisive Battles]