The first three divisions of troops to land then began a three pronged attack northward across the Korean countryside and advanced rapidly towards the capital of Seoul. They met very little resistance until they reached Ch'ungju, neary two-thirds of the way to their destination. Here they encountered a reorganized Korean army under the command of war hero General Sin Ip. Although the defenders fought valiantly, General Ip and his soldiers were wiped out. Their rusty swords were no match against the Japanese soldiers' muskets. The city fell quickly, and the invasion force continued towards Seoul unchecked. Upon receiving this news, King Sonjo abandoned his capital and fled towards Uiju on the Yalu River in northwestern Korea. The small contingent of troops he left behind for the defense of Seoul made a feeble stand along the Han River and was quickly overrun. Korea's capital of Seoul had fallen within three weeks of the start of the invasion.

It was at this point that Korean Admiral Yi Sun-sin began to make his presence known to the Japanese. In 1591 he had been appointed Left Navy Commander of Cholla Province, charged with protecting Korea's southwest coast. This was considered a very important post, for through these waters flowed the bulk of Korea's grain reserves destined for the large cities of the north. These shipments had been susceptible to Japanese pirate attacks for decades. Given the history of these attacks, and coupled with the possibility of a large-scale Japanese invasion, which Admiral Yi saw as imminent, he began to build up Korea's navy. Admiral Yi directed the construction of a fleet of modern warships, including one ship that he designed himself, unlike any that the world had ever seen. He called his invention kobuk-son, the turtle ship.