Korea eventually developed strong wooden ships called panokseons that made up the backbone of the Joseon navy. In the 15th century, under the decree of King Sejong, more powerful cannons were developed and tested. Used on battleships, the cannons proved to be a great success during actions against Japanese pirate ships. Panokseons and the Korean navy were most widely used during the Japanese invasions of Korea (1592-1598) when Admiral Yi Sun-sin's brilliant strategy defeated Japanese fleets. Admiral Yi also developed the turtle ship upon an older design.