Chinese scholars claim that Balhae was under the Tang dynasty's administrative control, because King Go of Balhae had received jarlig (imperial decree) as the Prefecture King of Balhae by Emperor Xuanzong.
The simple fact that King Go of Balhae received jarlig from the Tang dynasty is not sufficient evidence of the Tang dynasty's administrative control over Balhae. Just as with Balhae, Silla and Japan also received jarlig and paid tribute to the Tang dynasty. According to the logic of Chinese academia, Silla and Japan were also part of the Tang dynasty's administration. However, Chinese scholars are remarkably quiet in this regard. Perhaps they see the logical contradiction in their claims.
Balhae was an independent nation with aspects of an empire and even referred to its king as Emperor. In addition, people of Balhae who went to Tang to study abroad were able to take the Bin-gongke, which was an exam exclusively for foreigners.
Seo Hui and Deuk-gong Yu
When General Xiao Sunning of the Khitan Empire (Liao dynasty) invaded Goryeo in the 12th year of Seongjong (993 CE), Goryeo official Seo Hui recovered the 6th State of Gangdong through diplomatic negotiations. At this time, General Xiao Sunning claimed former Koguryo territories and expressed that Goryeo was occupying Khitan land. In response, General Seo Hui refuted this claim by stating that the country’s name Goryeo and its capital in Pyongyang were irrefutable proof that Goryeo had succeeded Koguryo.
In his seminal work, the Study of Balhae, Joseon era scholar Deuk-gong Yu recorded that after Koguryo and Baekje collapsed, Balhae and Silla occupied the territories to the north and south, respectively. This was collectively known as the North South States. He criticized the fact that Goryeo had not compiled a history of the North South States but also emphasized that Jo-yeong Dae (King Go) was of Koguryo descent, and the land occupied by Dae was Koguryo territory.