Korean history never lingers in peace for any length of time. In the 1500's, Emperor Hideyoshi of Japan was in search of new worlds to conquer. He was preparing to invade China and he requested that the Korean government give free passage to his army. The Koreans refused, so in 1592 AD the emperor retaliated by dispatching one-hundred and fifty-thousand infantrymen (armed with muskets) to invade Korea. His plan was to sweep through the peninsula and then conquer China. Seven-hundred soldiers of the Gumsan region fought the Japanese using mostly only Subak.
Korea was completely taken by surprise. Because of the decline of its military, it was not powerful enough to stop Japan, so Japanese troops began to advance north towards Seoul. The Koreans were outnumbered and daunted by muskets, so the Japanese invaders easily took Pusan and eventually Seoul.
However, the Korean people again refused to be conquered. Righteous armies of patriots, called "iybyon" (army of justice), composed of those who had kept the martial arts alive despite governmental disapproval, arose to defend their country. Using guerilla warfare tactics, they harassed the Japanese troops so effectively that they were forced to retreat to the south.
The most noted hero of the invasion was Admiral Yi Sun Sin for he was the man who played the greatest part in defeating the Japanese. A man of great ingenuity, he developed the kobukson "turtle ship," probably the first ironclad battleship in history. The galley was decked over with iron-clad panels to protect the rowers. The ship was so named because it was armed with a large, iron, ramming device in the shape of a turtlehead. These ships were almost impervious to any weapons that the Japanese could muster and so the ships were able to sink a large number of Japanese ships. To illustrate the effectiveness of these ships and the brilliant tactics of Admiral Yi, we need only look at one of the many battles he won. On September 16, 1597, he led 12 turtle ships against 133 Japanese ships in the Myongnyang Straits. The Koreans sank 31 enemy ships and sent the others fleeing in this victory. The Japanese were forced to give up their invasion (after seven years), but the struggle had devastated Korea. Whole towns had been looted and destroyed, crops were ruined, and famine and disease swept the country. After the war, the royal government revived strong defense measures by strengthening military training and martial arts practice.