One of the crucial incidents in Balhae history was Balhae’s attack on Tang Dengzhou in 732. This triggered the unified armed forces of Silla-Tang to engage in an attack on Balhae, similar to the Silla-Tang allied forces’ attack on Goguryeo in 668. How- ever, while Goguryeo had been no match for this powerful alliance, Balhae emerged victorious. In order to locate the factors that sustained Balhae’s durability and prosperity, this article explores the diplomatic relations between Silla, Balhae, and Tang, specifically by analyzing the sovereignty of Liaodong and the condition of the northwestern areas of the Korean peninsula where the three states bordered one another. Andong Protectorate, which originally ruled this area, was demoted to Andong Area Command in 698, making the northwestern Korean peninsula a buffer zone between Silla and Tang. Andong Protectorate was later restored in 705 only to be abolished again in 758, rendering Liaodong a buffer zone between Balhae and Tang. As a result, tension-inducing incidents with neighboring states, such as Balhae’s attack on Dengzhou, were not repeated, and a long period of peace between the three states of East Asia ensued.
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