North South States Period (698 CE – 926 CE) refers to the period in Korean history when Silla and Balhae coexisted in the south and north of the peninsula, respectively.
Main article: Unified Silla
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.
Korean arts flourished dramatically and Buddhism became a large part of Silla culture. Buddhist monasteries such as the Bulguksa are examples of advanced Korean architecture and Buddhist influence. State-sponsored art and architecture from this period include Hwangnyongsa Temple, Bunhwangsa Temple, and Seokguram Grotto, a World Heritage Site.
Silla began to experience political troubles in 780. 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.
Unified Silla lasted for 267 years until, under King Gyeongsun, it was annexed by Goryeo in 935.
Balhae was founded after Goguryeo had fallen. It was founded in the northern part of former lands of Goguryeo by Dae Joyeong, a former Goguryeo general. Balhae controlled the northern areas of the Korean Peninsula, much of Manchuria, and expanded into present-day Russian Maritime Province. Balhae styled itself as Goguryeo's successor state.
In a time of relative peace and stability in the region, Balhae flourished in culture, especially during the long reign of the third Emperor Mun (r. 737-793) and King Seon. At that time, Balhae was a culturally advanced country, so that even China referred to the Korean 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 Balhae and Silla language. Though Shoku Nihongi, an ancient Japanese record, implies the close relationship between the Balhae and Silla language: a student sent from Silla to Japan for interpreter training in Japanese language, assisted a diplomatic envoy from Balhae in communicating during the Japanese court audience.