United Silla 

Permanent wars between the Three Kingdoms (Koguryo, Paekche and Silla) for their territory was leading to creating a united state. In 654 AD Kim Chun-chu (金春秋) took the throne in Silla and strengthened relations with Chinese Tang Dynasty. As a result of this alliance, Tang and Silla invaded Paekche in 660 AD from the sea and land, and Koguryo was occupied in 688 AD. However, power in the two fallen states was in the hands of Tang. Because of that, Silla feared that next time she herself would loose independence and the whole Korean peninsula will become Tang's territory. Silla began to support guerrillas which was fighting against Chinese in the occupied Koguryo. At the same time, Silla moved to liberate former lands of Paekche. By 676 AD, Silla's troops had win the armies of Tang Dinasty. The Chinese Empire had to recognize Silla's border along Taedong River. So the territory to the South from Taedong River belonged to the United Silla, and all what laid to the North was absorbed by Tang's Empire. The United Silla created the basis for formation of the Korean nation with one language and culture. 

The King in the United Silla had unlimited power, but the state was governed by the Executive Council (Chipsason) and Internal affairs council (Naeson). For maintaining the centralized system, relatives of local authorities had to stay in the capital all the time, being given some posts, to prevent separatist movements.

Silla developed trade relations with Tang and Japan. 

By 9th century AD, the centralized state Silla had collapsed under peasant wars and separatism. Its territory was divided between small feudal states.