Sixth campaign and peace
The Mongols later learned that top Goryeo officials remained on Ganghwa Island, and had punished those who negotiated with the Mongols. Between 1253 and 1258, the Mongols under Jalairtai launched four devastating invasions in the final successful campaign against Korea.
Möngke realized that the hostage was not the blood prince of the Goryeo Dynasty. So Möngke blamed the Goryeo court for deceiving him and killing the family of Lee Hyeong, who was a pro-Mongol Korean general. Möngke' commander Jalairtai devastated much of Goryeo and took 206,800 captives in 1254. Famine and despair forced peasants to surrender to the Mongols. They established a chiliarchy office at Yonghung with local officials. Ordering defectors to build ships, the Mongols began attacking the coastal islands from 1255 onward. In the Liaodong Peninsula, the Mongols eventually massed Korean defectors into a colony of 5,000 households. In 1258, the king and the Choe clan retainer Kim Unjin staged a counter-coup, assassinated the head of the Choe family and sued for peace. When the Goryeo court sent the future king Wonjong as hostage to the Mongol court and promised to return to Kaegyong, the Mongols withdrew from Central Korea.
There were two parties within Goryeo: the literati party, which opposed the war with the Mongols, and the military junta — led by the Choe clan — which pressed for continuing the war. When the dictator Choe was murdered by the literati party, the peace treaty was concluded. The treaty permitted the maintenance of the sovereign power and traditional culture of Goryeo, implying that the Mongols gave up incorporating Goryeo under direct Mongolian control and were content to give Goryeo autonomy, but the king of Goryeo must marry a Mongolian princess and be subordinate to the Mongolian Khans.
Internal struggles within the royal court continued regarding the peace with the Mongols until 1270.
Since Choe Chung-heon, Goryeo had been a military dictatorship, ruled by the private army of the powerful Choe family. Some of these military officials formed the Sambyeolcho Rebellion (1270–1273) and resisted in the islands off the southern shore of the Korean peninsula.
Beginning with Wonjong, for approximately 80 years, Goryeo was a vassal state and compulsory ally of the Mongol Yuan Dynasty. The Mongols and Koreans rulers were also tied by marriages as some Mongol prince and aristocrats married Korean princesses and vice versa. During the reign of Kublai Khan, King Chungnyeol of Goryeo married one of Kublai's daughters. Later, a Korean princess called the Empress Gi became an empress through her marriage with Ukhaantu Khan, and her son, Biligtü Khan of Northern Yuan, became a Mongol Khan. The Kings of Goryeo held an important status like other important families of Mardin, Uyghurs and Mongols (Oirat, Hongirat, and Ikeres). It is claimed that one of Goryeo monarchs was the most beloved grandson of Kublai Khan and had grown up at the Yuan court.
The Mongol darughachis at the court of the Goryeo were offered provisions and sometimes were also willing to actively involved in the affairs of the Goryeo court. Part of Jeju Island converted to a grazing area for the Mongol cavalry stationed there. Even today, there are several Mongolian words used in the Jeju Island. Furthermore, the Mongol domination of Eurasia encouraged cultural exchange, and this would include for example the transmission of some of the Korean ideas and technology to other areas under Mongol control.
The Goryeo dynasty survived under influence of the Mongol Yuan Dynasty until it began to force Mongolian garrisons back starting in the 1350s, when the Yuan Dynasty was already beginning to crumble, suffering from massive rebellions in China. Taking advantage of the opportunity, the Goryeo king Gongmin also managed to regain some northern territories.