In 1592, on the command of Japanese Shogun Toyotomi Hideyoshi, approximately 200,000 Japanese soldiers invaded Korea, and the Waeran (Japanese War) began. At the beginning of the invasion, King Seonjo fled the capital, leaving a weak, poorly trained army to defend the country. In desperation he called on Seosan to organise monks into guerilla units. Even at 73 years of age he managed to recruit and deploy some 5,000 of these warrior monks, who enjoyed some instrumental successes. At first, the government armies of Joseon suffered repeated defeats, and the Japanese armies marched north up to Pyongyang and the Hamgyong provinces. At sea, however, the Joseon navy, under the command of Admiral Yi Sun-sin, enjoyed successive victories. Throughout the country, loyal volunteer armies formed and fought against the Japanese together with the warrior monks and the government armies of Joseon. The presence of Seosan's monks' army, operating out of the Heungguksa temple deep in the mountain of Yeongchwisan, was a critical factor in the eventual expulsion of the Japanese invaders in 1593 and again in 1598.