Japan invaded Korea after a series of long civil wars that had finally culminated in Toyotomi Hideyoshi leading a unified Japanese army. Their martial society had trained extensively with weapons like the arquebus, early guns, and the civil war had given them tremendous experience with siegecraft. By contrast, Korea had not been at war for hundreds of years: they were mostly troubled by raiders from without and corrupt government officials from within. However, their unique situation meant that they had great cannons for fighting off pirates and secure if minimal hill-top forts. As a tributary ally of the Chinese, the Koreans were startled and confused when Japan asked permission to march through their territory and make war on China. Many officials thought the Japanese were only bluffing, but Ryu Seong-ryong recognized the threat and made sure his friend Yi was moved to naval service to help defend the country. Yi trained his men and commissioned a new ironside ship design called the Turtle Ship. Unfortunately, other commanders did not take the threat seriously: even when Japanese ships appeared on the horizon, the southern commander convinced himself it was a trade fleet and took no action while the ships docked, then unloaded their soldiers onto Korean soil. The well-trained Japanese army crushed the Korean army and quickly advanced to Seoul. Meanwhile, Yi organized his small fleet of warships and launched quick strikes against the Japanese navy, catching them off-guard and on-patrol. He destroyed 43 enemy ships without losing a single one of his own, and was promoted accordingly to become the new Southern Commander of the Korean navy.