A series of campaigns were conducted between 1231 and 1270 by the Mongol Empire against the Korean kingdom of Goryeo. The last campaign concluded with a peace treaty with Goryeo becoming a vassal state of the Yuan dynasty, a relationship that lasted for approximately 80 years. Korea and the Song Dynasty of China put up a greater amount of "stubborn resistance" towards the Mongol invasions than many others in Eurasia who were swiftly crushed by the Mongols at a lightning pace.
The Yuan dynasty extracted wealth and tributes from the Goryeo kings. Despite their submission to the Yuan dynasty, internal conflicts among Goryeo's royalty and rebellions against Yuan rule continued, with the most notable being the Sambyeolcho Rebellion.
Khitan invasion (1216–1219)
Goryeo first encountered the Mongols in 1211 when a Goryeo envoy to the Jin dynasty (1115–1234) was killed by Mongol soldiers.
Later the Mongols entered Goryeo while pursuing enemy Khitans. In 1211, the Khitan prince Yelü Liuge who had been serving the Jin as a military commander rebelled and seized a portion of Liaodong. Two years later he proclaimed himself the ruler of the Liao. Yelü Liuge was ousted by his younger brother, Yelü Sibu, and requested help from Genghis Khan against the usurper. Sibu was also usurped by one of his ministers, Yelü Qinu. In 1214, the Jin assigned Puxian Wannu to suppress the rebellion. However Puxian Wannu was defeated and he himself rebelled in 1215. Based in Dongjing ("Eastern Capital"), Puxian Wannu declared the state of Dazhen ("Great Jurchen"). In the spring of 1216, Khitans fleeing from the Jin overran Puxian Wannu's territory and held the area from Dengzhou to Poju (Uiju).
In 1216, the Mongols accompanied by Yelü Liuge chased the Khitan rebels to the Goryeo borders and launched an attack on Dafuying, located on an island in the lower course of the Yalu River. The Khitans requested help from Goryeo but when the request was denied, the Khitans crossed the Yalu with an estimated 90,000 men and overran the Goryeo frontier. The Khitan rebels spent 1217 pillaging southwards down the Korean Peninsula before several defeats at the hand of Korean General Kim Ch'wiryŏ forced them to retreat. They turned their attention back toward the territory of Puxian Wannu. Puxian Wannu had already been defeated by the Mongols and submitted, but when the Mongol forces withdrew, he moved further east toward the Yalu and in early 1217, declared the state of Dongxia ("Eastern Xia"). Pressured by the Jin, he fled eastward again to the lower reaches of the Tumen River. It was also there that the Khitans fled from Goryeo. The Khitans were able to gather reinforcements and then invaded Goryeo again in the fall of 1217. The Khitan invasion was halted after they took the city of Kangdong, where Goryeo forces managed to contain them.
In the winter of 1218, 10,000 Mongol troops commanded by Hazhen and Zhala accompanied by 20,000 Eastern Jurchen troops commanded by Wanyan Ziyuan entered Goryeo from the northeast. They defeated Khitan forces in the cities of Hwaju, Maengju, Sunju, and Tŏkchu. The Mongol-Jurchen advance was stopped by a heavy snowfall that made the roads impassable. Hazhen sent a letter carried by the translator Zhao Zhongxiang to Cho Ch'ung, the Goryeo commander in charge of the northwest, requesting provisions and demanding the two nations enter an Elder-Younger Brother relationship after the subjugation of the Khitans.
Cho Ch'ung and Kim Ch'wiryŏ were in favor of meeting the Mongol demands but the Goryeo court was more apprehensive. Ultimately they agreed to provide for the Mongol forces at the urging of Cho. One thousand bushels of rice and one thousand picked troops were sent to the Mongols. They arrived in time to witness the Mongols assault the Khitans in the walled-city of Taeju. The reinforcements were received well by the Mongols.
In early 1219, preparations were underway to take the last Khitan stronghold in Kangdong. Troops from Goryeo joined the Mongol-Jurchen force. An estimated 50,000 Khitans surrendered and opened the gates.
After the fall of Kangdong, Goryeo sent a delegation to Zhala's camp as well as gifts to the Mongol commanders. Mongol envoys met with Gojong of Goryeo (reigned 1213–1259) and handed him a document without the usual formalities. Cho accompanied the Mongol and Jurchen commanders to the Yalu where the Mongols seized a large number of Goryeo horses and left. However they left behind 41 subordinates in Poju to learn the language of Goryeo and to wait for the Mongols' return. The Mongols also started collecting annual tribute from Goryeo and within two years, Gojong was advocating for the cessation of tribute payments and resistance against the Mongols.
Conditions in the northwest were poor following the Khitan raids. In the autumn of 1219, military commanders Han Sun and Ta Chi rebelled in Poju and defected to Puxian Wannu, who augmented their forces with 10,000 Eastern Jurchens. They tried to make an alliance with the Jin commander Yugexia. Yugexia invited them to a feast and killed them. Their heads were sent to Kaesong in early 1220 for which Yugexia was rewarded greatly by the Goryeo court. However Yugexia repeatedly sacked Goryeo's border cities for several years afterward. Troops sent to Poju to restore order slew so many that another rebellion occurred four months later that required 5,000 troops to put down. Khitans who had fled into the mountains continued to raid Goryeo garrisons.
First Mongol invasion of Korea (August 1231 – January 1232)
In 1221, a Mongol delegation led by Zhuguyu made a list of demands while 6,000-7,000 Mongol troops arrived at the Goryeo border a few days later. They were received coldly by the Goryeo court. In 1224, Puxian Wannu declared independence from the Mongols and sent envoys to Goryeo to establish an alliance. Goryeo rejected the offer and over the next four years, Eastern Jurchens raided Goryeo. An attempt at peace negotiations was made in 1229 due to deteriorating relations between the Mongols and Goryeo.
A Mongol delegation led by Zhuguyu had arrived in 1224 to supervise the transfer of tribute. They took only the otter pelts but discarded the other items in the fields while crossing the Yalu. During their journey they were killed by bandits. Since the Mongols suspected Goryeo, relations had been terminated in early 1225. When the Mongols ordered Goryeo to attack Puxian Wannu, Goryeo did not comply.
On 26 August 1231, a Mongol force led by Saritai crossed the Yalu and surrounded Hamsin-chin (Uiju). Defense General Cho Sukch'ang and Deputy Commissioner Chŏn Kan surrendered Hamsin-chin to the Mongols. The Mongol forces split with one group heading north to attack Sakju while the other group headed down the Yalu. When the second group heading south reached Chongju, Kim Kyŏngson at first tried to fight the Mongols but the entire city had already fled, so he retreated to Kuju Castle. From Chongju, the Mongols proceeded to Inju where Hong Pok-wŏn defected and submitted the area and 1,500 households. His family later played a pivotal role in the Mongol campaigns against Goryeo by acting as guides. At Ch'ŏlchu (in Hamgyong Province), the men slit their throats and burned the granary and all the women and children rather than submit to the Mongols.Yongju, Sonju, and Kwakchu fell in October.
The Mongols bypassed Chaju and the Western Capital (Pyongyang) where they encountered stiff resistance. The slave army led by Ji Gwang-su fought to the death. They took Hwangju before reaching Chabi Pass, the gateway to Kaesong.
Ch'oe U called up the Three Armies and levied troops from the provinces. Yugexia contributed 5,000 soldiers and even bandits entered the Goryeo army. The Three Armies under the leadership of Yi Chasŏng departed Kaesong in early October and fended off a surprise attack by 8,000 Mongol soldiers. The Goryeo army was defeated at Anju, after which Goryeo started negotiations with the Mongols. However, when news that the Mongols demanded Goryeo's submission reached the court in late November, the Goryeo response was to continue resistance. Kaesong was surrounded by Mongol forces in late December. Goryeo officially submitted and the Three Armies surrendered.
Goryeo's surrender did not mean an immediate end to the fighting. The Mongols sacked cities which they had previously taken. Chŏn Kan attempted to kill the Mongols in Hamsin-chin but a certain Xiaoweisheng escaped. Chaju held out while heavy fighting occurred at Chongju. At the Siege of Kuju, Pak Sŏ led forces who had gathered from surrounding cities in a defense against the Mongols. Kim Chungon defended the eastern and western walls while Kim Kyŏngson defended the south. The Mongols attacked the west, south, and north gates repeatedly. They used carts of grass and wood as well as towers to try to scale the walls but the defenders countered with molten iron, setting fire to the siege machines. They also tried to mine the city walls but the tunnel collapsed. The south wall was attacked by 15 large catapults. The defenders used their own catapults to drive off the attackers while using mud and water to put out fires by inflammatory projectiles. The Mongols withdrew for a time after 40 days of besieging the city only to return with reinforcements. They set up 30 catapults and breached the city wall in 50 places but they were repaired while Pak Sŏ made a successful sortie and drove off the attackers. Kuju and Chaju finally submitted after multiple orders from the Goryeo court. Pak Sŏ and Ch'oe Ch'unmyŏmg, defender of Chaju, were almost executed but the Mongols spared them owing to their courage, which they admired.
Saritai demanded 10,000 small horses, 10,000 large horses, 10,000 bolts of purple gauze, 20,000 otter skins, and clothing for a million men army. Hostages from the king's family, provincial leaders, and high officials were also demanded. A large tribute was presented by Goryeo but no hostages. General Saritai began withdrawing his main force to the north in the spring of 1232, leaving 72 Mongol administrative officials (Darughachi) stationed in various cities in northwestern Goryeo to ensure that Goryeo kept to the peace terms. They controlled almost all of modern North Pyongan Province and a great portion of South Pyongan Province as well.
Second Mongol invasion of Korea (June 1232 – December 1232)
When the Mongols threatened Kaesong in the previous invasion, Ch'oe U sent his family to Ganghwa Island. In the spring of 1232, Ch'oe U gathered all the high officials at his residence and suggested transferring the capital to Ganghwa. A dissenter, Kim Sech'ung, was executed for speaking out against the plan. The royal family relocated to Ganghwa and commissioners were sent out to the provinces to instruct the people to take refuge in mountain citadels or coastal islands. With the transfer complete, in the summer of 1232, the Mongol darughachis on the mainland were killed.
The Western Capital (Pyongyang) was still under the control of Hong Pok-wŏn, a Goryeo warlord who had defected to the Mongols. When the people of the Western Capital learned that the court was planning on killing the Mongol officials, they rebelled out of fear that Mongol reprisal would result in their deaths. Government officials in the Western Capital fled to Chŏ Island.
In the summer of 1232, the Mongol invasion led by Saritai advanced south into the Han valley without much resistance. However in an attack on Ch'ŏin-song, Saritai was killed by an arrow shot by the monk Kim Yun-hu. The Mongol army retreated, leaving Hong Pok-wŏn to supervise the territory that they had conquered.