[27] This is considered by modern Korean scholars to be Goryeo's Northern Expansion Doctrine (북진 정책) to reclaim ancestral lands formerly owned by Goguryeo.

[44][45][46] Beginning in the late 8th century, Later Silla was undermined by instability because of political turbulence in the capital and class rigidity in the bone-rank system, leading to the weakening of the central government and the rise of the "hojok" (호족; 豪族) regional lords.

[52][53][54] Wang Kŏn entered military service under Kung Ye at the age of 19 in 896, before Later Goguryeo had been established, and over the years accumulated a series of victories over Later Baekje and gained the public's confidence.

[62] In 935, Kyŏn Hwŏn was removed from his throne by his eldest son over a succession dispute and imprisoned at Geumsansa Temple, but he escaped to Goryeo three months later and was deferentially received by his former archrival.

In addition, Taejo included the Balhae crown prince in the Goryeo royal family, unifying the two successor states of Goguryeo and, according to Korean historians, achieving a "true national unification" of Korea.

[72][h] Due to this constant massive influx of Balhae refugees, the Goguryeoic population in Goryeo is speculated to have become dominant[74][75][76][77] in proportion compared to their Silla and Baekje counterparts that have experienced devastating war and political strife[78][79][80] since the advent of the Later Three Kingdoms.

In 956, Gwangjong freed the prisoners of war and refugees who had been enslaved by the hojok during the tumultuous Later Three Kingdoms period, in effect decreasing the power and influence of the regional nobility and increasing the population liable for taxation by the central government.

[90] In contrast to Goryeo's traditional "dual royal/imperial structure under which the ruler was at once king, emperor and Son of Heaven", according to Remco E. Breuker, Gwangjong used a "full-blown imperial system".

[92] Seongjong filled the bureaucracy with new bureaucrats, who as products of the gwageo civil service examinations were educated to be loyal to the state, and dispatched centrally-appointed officials to administrate the provinces.

The Goryeo commander Kang Kam-ch'an had dammed a large tributary of the Yalu River and released the water on the unsuspecting Khitan soldiers, who were then charged by 12,000 elite cavalry.

[108][116] The History of Liao claims that Hyeonjong "surrendered" and Shengzong "pardoned" him, but according to Hans Bielenstein, "[s]horn of its dynastic language, this means no more than that the two states concluded peace as equal partners (formalized in 1022)".

[117] Kaesong was rebuilt, grander than before,[9] and, from 1033 to 1044, the Cheolli Jangseong, a wall stretching from the mouth of the Yalu River to the east coast of the Korean Peninsula, was built for defense against future invasions.

[145] Goryeo was a period of great achievements in Korean art and culture, such as Koryŏ celadon, which was highly praised in the Song dynasty,[25][146] and the Tripitaka Koreana, which was described by UNESCO as "one of the most important and most complete corpus of Buddhist doctrinal texts in the world", with the original 81,258 engraved printing blocks still preserved at Haeinsa Temple.

[157][158] Yun Kwan believed that the loss was due to their inferior cavalry and proposed to the king that an elite force known as the Byeolmuban (別武班; "Special Warfare Army") be created.

According to Breuker, Goryeo never really had control of the region occupied by the Nine Fortresses in the first place and maintaining hegemony would have meant a prolonged conflict with militarily superior Jurchen troops that would prove very costly.

[171] In addition, under the reign of King Uijong, military officers were prohibited from entering the Security Council, and even at times of state emergency, they were not allowed to assume commands.

[183] Although the House of Ch'oe established strong private individuals loyal to it, continuous invasion by the Mongols ravaged the whole land, resulting in a weakened defense ability, and also the power of the military regime waned.

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 against Korea at tremendous cost to civilian lives throughout the Korean peninsula.

One major issue that had long fermented in Chinese Seon, and which received special focus from Jinul, was the relationship between "gradual" and "sudden" methods in practice and enlightenment.

Drawing upon various Chinese treatments of this topic, most importantly those by Zongmi (780–841) and Dahui (大慧; 1089–1163),[223] Jinul created a "sudden enlightenment followed by gradual practice" dictum, which he outlined in a few relatively concise and accessible texts.

Three important monks of this period who figured prominently in charting the future course of Korean Seon were contemporaries and friends: Gyeonghan Baeg'un (景閑白雲; 1298–1374), Taego Bou (太古普愚; 1301–1382) and Naong Hyegeun (懶翁慧勤; 1320–1376).

The Mongols had acquired the technique of distilling Arak from the Muslim world[238] during their invasion of Central Asia and the Middle East around 1256, it was subsequently introduced to Koreans and distilleries were set up around the city of Kaesong.

This script originates in Goguryeo and was designed for a very specific sort of syntax that made use of postpositional particles, such as the Old Korean accusative marker *-ul/*-ur ending represented by 尸 'corpse' and 乙 '2nd Heavenly stem'.

[citation needed] During the Goryeo period, lacquerware with mother-of-pearl inlay reached a high point of technical and aesthetic achievement and was widely used by members of the aristocracy for Buddhist ritual implements and vessels, as well as horse saddles and royal carriages.

Although Korean lacquerware of the Goryeo period was highly prized throughout East Asia, fewer than fifteen examples are known to have survived, one of which is this exquisite box in the Museum's collection.

This paucity of material is largely attributable to the fragility of lacquer objects and, to a certain extent, to wars and raids by foreign powers, notably those launched from Japan by Toyotomi Hideyoshi (1536–1598) in the late sixteenth century.

The body is low clay, quartz rich, high potassia and virtually identical in composition to the Chinese Yueh ceramics which scholars hypothesize occasioned the first production of celadon in Korea.

The distinctive blue-grey-green of Korean celadon is caused by the iron content of the glaze with a minimum of titanium contaminant, which modifies the color to a greener cast, as can be seen in Chinese Yueh wares.

Scholars also theorize that this developed in part to an inlay tradition in Korean metalworks and lacquer, and also to the dissatisfaction with the nearly invisible effect of incising when done under a thick celadon glaze.

"With his lieges assembled in the Cheondeok Palace, the King spoke out: For long have I vowed my devotion towards our alliance and friendship with Silla by painting my lips with blood as an oath to preserve our royal lines together.