After the unification wars, the Tang Dynasty established territories in the former Goguryeo, and began to administer and establish communities in Baekje. Silla attacked the Chinese in Baekje and northern Korea in 671.
The Tang Dynasty then invaded Silla in 674 but Silla defeated the Tang army in the north. Silla drove the Tang forces out of the peninsula by 676 to achieve unification of most of the Three Kingdoms.
Silla began to experience political troubles in the late 9th century. This severely weakened Silla and soon thereafter, descendants of the former Baekje established Later Baekje. In the north, rebels revived Goguryeo, beginning the Later Three Kingdoms period.
In a time of relative peace and stability in the region, Balhae flourished in culture, especially during the long reign of the third King Mun (r. 737-793) and King Seon. At that time, Balhae was a culturally advanced country, so that even China referred to this kingdom as "a prosperous country of the East." However, Balhae was severely weakened by the 10th century, and the Khitan Liao Dynasty conquered Balhae in 926.
Goryeo absorbed some of Balhae's territory and received Balhae refugees, including the crown prince and the royal family, but compiled no known histories of Balhae. The 18th century Joseon dynasty historian Yu Deukgong advocated the proper study of Balhae as part of Korean history, and coined the term "North and South States Period" to refer to this era.
Due to the lack of linguistic evidence, it is difficult to make a definitive conclusion for the linguistic relation between the Balhae and Silla languages.
Shoku Nihongi implies that the Balhae language, a Goguryeo language, and Silla language share a close relationship[clarification needed]: a student sent from Silla to Japan for an interpreter training in Japanese language assisted a diplomatic envoy from Balhae in communicating during the Japanese court audience.
One terminology that people of Balhae used to describe "a king" is Gadokbu (transcribed as 可毒夫).
^Reischauer, Edwin Oldfather. Ennins Travels in Tang China. John Wiley & Sons Canada, Limited. pp. 276–283. ISBN9780471070535. Retrieved 21 July 2016. "From what Ennin tells us, it seems that commerce between East China, Korea and Japan was, for the most part, in the hands of men from Silla. Here in the relatively dangerous waters on the eastern fringes of the world, they performed the same functions as did the traders of the placid Mediterranean on the western fringes. This is a historical fact of considerable significance but one which has received virtually no attention in the standard historical compilations of that period or in the modern books based on these sources. . . . While there were limits to the influence of the Koreans along the eastern coast of China, there can be no doubt of their dominance over the waters off these shores. . . . The days of Korean maritime dominance in the Far East actually were numbered, but in Ennin's time the men of Silla were still the masters of the seas in their part of the world."