Koryo is a feudal state which existed between 918 and 1392 in the Korean history. The capital was Kaesong today. The founder was Wang Kon.
He named his country Koryo in the meaning that it was succeeding to the Koguryo Dynasty which was well-known as a 1,000 year-long power in the East while existing between 227 BC and AD 668. Carrying forward the aspiration of Koguryo to merge different countries of fellow tribes, Wang Kon waged an active struggle for territorial unification.
At that time in the Korean peninsula there were Koryo, Later Paekje and Silla south of the River Taedong flowing in the middle west and Palhae north of the river.
Wang Kon enforced the policy of winning over Silla, taking a strong attitude towards Later Paekje and actively embracing the Palhae people.
At that time the territory of Silla was divided and its might extremely declined due to the separatist moves of the feudal forces and uninterrupted peasants’ wars. Under such circumstances, Koryo positively enforced the appeasement policy of merging Silla and forced it to surrender in 935.
As for Later Paekje which was the main target of Koryo, he merged it by force of arms in 936, taking advantage of the scramble of the rulers for power.
Through a complicated struggle for 18 years after founding, Koryo could unify the countries of fellow tribes.
After the fall of Palhae in 926, Koryo embraced its people and took the northwestern region which was land of Koguryo, expanding the territory.
Koryo fixed Pyongyang which was the capital of Koguryo as its western capital and built fortresses in the northwestern region around it. Until the 980s after its founding Koryo built, enlarged or repaired a total of 50 fortresses in the north of the River Taedong.
After completely taking the northwestern region, Koryo also persistently struggled to take back the northeastern region.
It dispatched troops to the northeastern region several times between the early 12th century and the latter half of the 14th century to deal military blows at the foreign invaders and continued to expand the territory.
So the territory of Koryo included almost all of the territory of Korea today.