Goryeo under Mongol rule

From about 1270 to 1356, the Korean kingdom of Goryeo was ruled by the Mongol Empire and the Mongol-led Yuan dynasty. After the Mongol invasions of Korea and the capitulation of Goryeo in the 13th century, Goryeo became a semi-autonomous vassal state and compulsory ally of the Yuan dynasty for about 80 years. It has been referred to as a "son-in-law kingdom in the Mongol Empire." The ruling line of Goryeo, the House of Wang, was permitted to rule Korea as a vassal of the Yuan, which established the Branch Secretariat for Eastern Campaigns (征東行省; literally "Branch Secretariat for Conquering the East") in Korea as an extension of Mongol supervision and political power. Members of the Goryeo royal family were taken to Khanbaliq, and typically married to spouses from the Yuan imperial clan, the House of Borjigin. As a result, princes who became monarchs of Goryeo during this period were effectively imperial sons in-law (khuregen). Yuan overlordship ended in the 1350s when the Yuan dynasty itself started to crumble and King Gongmin of Goryeo began to push the Yuan garrisons back.

The Mongol Empire first made contact with the Korean kingdom of Goryeo when they killed a Goryeo envoy to the Jin dynasty in 1211. In 1216, a host of Khitans fleeing the Mongols crossed into Goryeo. A Mongol-Jurchen army arrived on Goryeo's borders two years later chasing after the enemy Khitans. After defeating the Khitans with the help of Goryeo, the Mongol army left behind dozens of overseers to learn the Korean language. From 1219 to 1224, the Mongols sent envoys to collect tribute from Goryeo. In 1225, a Mongol envoy was killed under uncertain circumstances while returning from a tribute collecting mission. The Goryeo officials blamed the murder on a bandit but the Mongols believed that the Goryeo government was responsible. Moreover, the Mongols were displeased with Goryeo's refusal to contribute military forces for the Mongol campaign against the Eastern Jurchens. They launched several invasions against Goryeo from 1231 to 1259. There were six major campaigns: 1231, 1232, 1235, 1238, 1247, 1253; between 1253 and 1258, the Mongols under Möngke Khan's general Jalairtai Qorchi launched four devastating invasions in the final successful campaign against Korea, at tremendous cost to civilian lives throughout the Korean Peninsula. The Mongols annexed the northern areas of the Korean Peninsula and incorporated them into their empire as Ssangseong Prefecture and Dongnyeong Prefecture.

Ögedei Khan dispatched Sartaq against Goryeo and after they ravaged the Korean countryside, Goryeo accepted the placement of overseers known as darughachi within its borders. At the time, Goryeo was ruled by a military regime led by Ch'oe U (r. 1219–1249). The Ch'oe family had seized power from the king in the late 12th century. Despite retaining the king on the throne and the existing administrative structures, the Ch'oe family was usually the ones determining policy. In June 1232, Ch'oe U moved the court from Gaegyeong to the more defensible Ganghwa Island and murdered all the darughachi overseers. Ch'oe was willing to send tribute but refused to accept overseers, send royal hostages, or return the court to Gaegyeong. Further campaigns against Goryeo by the Mongols were undertaken by Tanggud (1253–1254), Ebügen (1247–1248), Prince Yekü (1253–1254), and Jalairtai (1254–1255). Goryeo quickly discovered that pitched battles usually led to defeat and instead retreated into the mountain fortresses and islands where they could more effectively defend their positions with their greater understanding of local geography. In 1241, Goryeo sent Wang Sun (1224–1283), a distant relative of the royal family posing as the crown prince, as hostage to the Mongols. Devastation from the Mongol raids caused the peasants to defect to the Mongols, who established Ssangseong Prefecture with the aid of local officials.

Goryeo held out against the Mongol invasions for about thirty years. Eventually, due to increasing pressure and instability from repeated devastation caused by the Mongol invasions, the Ch'oe family was ousted from power. In May 1258, Ch'oe Ui (last head of the Ch'oe family) was assassinated by Kim Chun, ending the Ch'oe military regime and returning the king to power. For the next ten months, the new government debated on how to handle the Mongol invasions, with the majority favoring a settlement agreement. In May 1259, the crown prince Wang Chŏn was sent as a hostage to the Mongol court where he agreed to destroy Ganghwa's fortifications and return the capital to the mainland. Upon receiving news that his father had died, Wang Chŏn was sent back to Goryeo where he was enthroned as king (posthumously Wonjong of Goryeo) in June 1260. Shortly after, he received a number of edicts from Kublai Khan insisting again on the return of the capital to the mainland, but also agreeing to release thousands of Goryeo captives and that Goryeo may keep their traditional clothing. The latter command was later interpreted as an overarching directive to preserve Goryeo's institutions such as slavery and land tenure practices.

Wonjong's government led by Kim Chun was overthrown by Im Yŏn in 1269. In response, the Mongols backed the rebellion of another group of Goryeo officials in the northwest and created the Dongnyeong Prefecture. Another invasion was prepared in 1270. In an event known as the Sambyeolcho Rebellion, the Three Patrols army (sambyeolcho) that served Goryeo's government rebelled against the Im family, overthrew them, and moved the officials back to Gaegyeong from Ganghwa. They fled to Jindo Island and then Jeju Island, where they remained until 1273 when Goryeo forces arrived and defeated them, after which a part of the island was converted to a breeding ground for the Yuan royal herd. Goryeo regained formal control of Dongnyeong in 1290 and Jeju in 1294. However the military command (Tumen) on Jeju (Tamna Prefectures) remained outside of their jurisdiction.

Due to the turmoil caused by the Mongol invasions, a number of Koreans from northern Goryeo entered China either as captives or willingly to seek their fortunes elsewhere, especially in the Yuan capitals of Khanbaliq and Shangdu. Looser control from the Goryeo government resulted in the departure of farming families for Liaoyang and Shenyang to escape tax and labor services. Possibly as many as 250,000 Koreans lived in China during this period.

Militarily, following the 1259 peace treaty, Mongol ambitions on Japan resulted in two invasions of Japan. In both efforts, the Mongols directed Korean shipbuilding and militarization towards the amphibious assault of the Japanese coasts and pressed a large proportion of Korean naval and infantry forces into the service of Mongol military objectives. Korea supplied 770 fully manned ships and 5,000 soldiers in 1274 and 900 ships and 10,000 soldiers in 1281. During the preparations for the invasion of Japan, Kublai established two institutions to oversee affairs in Goryeo. An office for military agricultural colonies was created in 1271 and the Branch Office for Eastern Campaigns was created in 1280. The former office was abandoned soon after but the Branch Secretariat continued to function as a feature of Mongol-Goryeo administration for the next 70 years. Yuan officials and envoys took concubines and wives in Korea while they were stationed in Korea for the invasion of Japan. For a variety of reasons, both invasions failed. During the periods leading up to and during the invasions, Korea was effectively forced to serve as a Mongol military base. The Yuan dynasty paid for ships and soldiers in Goryeo with baochao paper money.

After 1270, Goryeo became a "fully integrated client kingdom," however official protocol was that of a subordinate principality. David M. Robinson described Goryeo's status as incorporated within the "Great Yuan ulus" but legally distinct from the rest of the empire. Yuan legal codes separated Goryeo from southern China. In the 1990s, some Korean scholars called the period of Mongol rule the age of "Yuan intervention". Rashid al-Din Hamadani described Goryeo as a province in name but a separate state within the Yuan dynasty. It has also been called a "son-in-law kingdom in the Mongol empire".