Silla was torn to pieces by rebel leaders such as Gyeon Hwon who proclaimed the Latter Baekje (Hu Baekje) state in Jeonju in 900, and Gung Ye who proclaimed the Latter Goguryero (Hu Goguryeo) state, the following year at Gaeseong. Wang Geon, the last rebel leader, the son of a gentry family, became the first minister of Gung Ye. Overthrowing Gung Ye for misdemeanors and malpractice in 918, he sought and received the support of landlords and merchants whose economic as well as political power overwhelmed the Silla government.

Wang Geon easily raided Latter Baekje in 934. Wang Geon accepted the abdication of King Gyeongsun of Silla in 935. The following year he easily conquered Latter Baekje and unified the Korean Peninsula.

Wang Geon was at first content to leave provincial magnates undisturbed. He was particularly careful to placate the Silla aristocracy. He gave former King Gyeongsun the highest post in his government, and even married a woman of the Silla royal clan, thus somewhat legitimizing his rule.

Enthroned as the founder king of the Goryeo Dynasty (918-1392), the name of which was derived from Goguryeo, he drafted 10 injunctions for his successors to observe. Among the 10 injunctions he predicted probable conflict between his state and the northern nomadic states with Goguryeo's territory as the objective, and advised the strengthening of the state. He advised that Buddhist temples must not be interfered with, and warned against the usurpation and internal conflicts among the royal clans and the weakening of local power.

King Taejo's (Wang Geon's posthumous title) lenient policy plus his marriage ties made the rebellious local lords relatively obedient. To weaken the local power, King Gwangjong (r.949-975) instituted emancipation of slaves in 956 in order to restore the commoner status of those unjustly bonded. This helped to increase revenue and was welcomed by the people unjustly forced into captivity.

Two years later, he installed a civil service examination system to recruit officials by merit. His successor King Gyeongjong (r.975-981), put into practice the allotting of land and forest lots to officials. These policies enabled the Goryeo Dynasty to gain a foothold as a centralized government. King Seongjong (r.981-997) in 982 adopted the suggestions in the memorial written by Confucian scholar Choe Seungro and paved the way to rule by Confucian state model. District officials were appointed by the central government, and all arms privately owned were collected to be recast into agricultural tools.

The government organization was set up after the Tang system, but the power to make admonitions to the throne on the part of officials and censorship of royal decisions was instituted. With such internal order, Goryeo was long able to withstand foreign invasion.

The Khitan rose to power and began to confederate, transforming their old tribal league into a centralized organization. They conquered Balhae in 926 and, officially came to be called Liao in 938. As noted earlier, the people of Balhae fled to Goryeo, but Liao was now ready to strike, and Goryeo tried in vain to open diplomatic relations. Liao initiated attacks in 983, in 985, in 989, and in 993, continuing to harass Goryeo. However, in 993, Goryeo's commanding general Seo Hui (940-998), facing a stalemate with the Liao army, convened peace talks with Liao general Hsiao to end the enmity with the recognition of Goryeo's territorial rights of south of the Amnokgang river.

Diplomatic relations were opened between the two states in 994. But Liao attacked again in 1010 and the Goryeo king fled to the south. The conflict became more complicated as the northern Jurchen tribes grew stronger in the Korean border area of Manchuria. As the conflicts continued to afflict war-weary Goryeo, King Hyeonjong (r.1009-1031) ordered the carving of the Tripitaka, imploring Buddha's aid, which consisted of about 6,000 chapters.

However, in 1115 the Jurchen established the Jin Empire and came into conflict with Liao. Jin conquered Liao in 1125, and turned to an invasion of Song. By 1126 it conquered the Northern Song which fled south of the Yangtze River. Two Song emperors were captured by Jin, and royal as well as private Song libraries came into Jin possession.

Goryeo had its own calamity that year. In 1126, all of the palace buildings, including tens of thousands of books in the royal library and national academy, went up in smoke when the palace buildings were set afire by the father-in-law of King Injong. Goryeo lost the famed collection, and there was no way to obtain books from the Song. To print books with wood blocks was prohibitive in cost and time consuming. Then came the idea of typography and the casting of bronze type began with the same technology that was used in coin-casting. Goryeo printing with movable metal type was developed to print many titles in limited copies around the mid-12th century.

In 1145, King Injong (r.1112-1146) had a Confucian scholar, Kim Busik, compile the Samguk sagi (History of the Three Kingdoms). About one hundred years later, a monk by the name of Iryeon compiled the Samguk yusa (Memorabilia of the Three Kingdoms), which records important history and traditions that are not found in the Samguk sagi.

Conflict increased between civil and military officials as the latter were degraded and paid poorly. In 1170, the military officials rose up against the civil officials and paid them back with bloodshed. Around this time the Mongols consolidated power, and the new Song techniques of smelting iron with corks was utilized by the Mongols in the production of arms. With the new arms, the Mongols conquered Jin in 1215 and chased the diehard Liao refugees into the territory of Goryeo, which was consequently plagued by consecutive Mongol invasions. As a result, the Goryeo court and officials fled to Ganghwado Island in 1232.

Mongols invaded in 1238 and looted Goryeo, destroying the splendid Silla pagoda of Hwangnyongsa Temple. The Goryeo court on Ganghwado Island carved the second Tripitaka Koreana consisting of over 80,000 wood blocks inscribed on both sides, which is now stored at Haeinsa Temple. This enormous task was also conducted with pious patriotism to secure Buddha's protection against the Mongols. The people of Goryeo reached a consensus to resist the foreign invaders and safeguard the nation despite the incessant attacks and invasions.

From the middle of the 14th century, the Mongol power declined rapidly, with their own internal struggles for the throne, and in the 1340s, frequent rebellions broke out all over China.

Freed at last from Mongol domination, Goryeo began efforts to reform its government. King Gongmin (r.1351-1374), first removed pro-Mongol aristocrats and military officers. These deposed people formed a dissident faction which plotted an unsuccessful coup against the king.

A second internal problem was the question of land holdings. By now the land-grant system had broken down, and Mongol-favored officials and military men, along with a handful of landed gentry, owned the vast majority of agricultural land, which was worked by tenant farmers and bondsmen. King Gongmin's attempt at land reform was met with opposition and subterfuge from those officials who were supposed to implement his reforms, as they were landowners and the policy of land ownership was supposed to undergo a drastic change.

A third problem was the rising animosity between the Buddhists and Confucian scholars. Normally, and during most of the dynastic period, Buddhism and Confucian creeds coexisted with little conflict. It must be noted here that by this time, Korean scholars had become imbued with the Neo-Confucian doctrine as advocated by Zhu Xi in the late 12th century, just before the advent of the Mongols. The new Confucian scholars did not agree with the idea that one should denounce one's family ties to become a monk because the very basis of Confucian philosophy was founded on strong family and social relationships. The wealth and power of the monasteries and the great expense incurred by the state for Buddhist festivals became a major target of criticism.

Another problem was that Japanese pirates were no longer hit-and-run bandits, but organized military marauders raiding deep into the country. It was at that time that General Yi Seonggye distinguished himself by repelling the pirates in a series of successful engagements.