Facing invasion by the Mongol Empire in 1231, Goryeo immediately relocated its capital from Gaegyeong to Ganghwa Island and moved its people there for safety (called “ip-bo”, 入保). Goryeo’s move to Ganghwa Island exploited the Mongols weakness: attacking over water. As the relocation mainly aimed to secure the sea route by using the island as a strategic foothold with which repel the Mongols, the strategy can be called the “Island-Marine Military Strategy”. The initial Mongolian military strategy had ravaged parts of the Korean peninsula. The second military operation, launched in1250s, focused on attacking the coastal areas and islands of Goryeo to little effect. Thus, the Mongolians intensified the pressure on Goryeo by alternating between armed incursions and peace talks. The Goryeo scholars who had insisted on peace with Mongolia gained greater influence, and Goryeo struck peace with the Mongo Empire in 1270, whereby the capital was moved back to Gaegyeong.
However, some military officials, in what was known as the “Sambyeolcho” (a special capital defense unit), refused to surrender and relocated to certain islands (Jin Island and Jeju Island) off the southern shore of the Korean peninsula. They brought the nearby islands and coastal regions under their control and continued to resist the Mongolians for another three years by re-launching the “Island-Marine Military Strategy.” The end to their resistance came with the capture of Jeju Island in 1273 by the Mongolians.
However, the Sambyeolcho forces and their sympathizers had not been completely annihilated. Some of the Sambyeolcho forces escaped from Jeju Island and reached Okinawa, while local forces stationed around other islands and coastal regions remained a considerable threat to the Mongolians and Goryeo. In response, the Mongols and Goryeo took the drastic action of ordering “Gongdo”(moving people off the islands) in order to drive out the remaining forces of the rebellion from the islands and coastal regions.
In conclusion, the Mongol invasion of Goryeo in the 13th century, which had served as the foundation for their war against East Asia to conquer the Southern Song Dynasty and Japan, led to “Gongdo” following an almost 40 year-long struggle. In retrospect, this move appears to have been a prelude to the imminent “Haegeum” (ban on maritime activities) and the other examples of “Gongdo” seen in East Asia in the 14th century.