Although the struggles amongst Goguryeo, Baekje, and Shilla ended in a victory for Shilla, as Allied forces of Shilla and Tang collapsed the former two kingdoms, there still remain two questions. First question is why there were a rupture and a war between Shilla and Tang after the falls of Baekje and Goguryeo. And the second one is why the Goguryeo people's revival movement resulted in the foundation of Balhae, while the Baekje people failed to revive their own.
To answer these two questions, certain aspects of the Tang's policy to rule old territories of Baekje and Goguryeo should be examined. And examination shows us that Tang's control over Baekje's old territory was more severe than that over Goguryeo's. In addition, Shilla as well had a different attitude toward the two regions. Shilla concentrated more on Baekje, in terms of the size of stationary troops or the victims(prisoners) of the unification war.
This kind of difference resulted from the precondition of the alliance arranged between Shilla and Tang. They were to gain the old territories of Baekje and Goguryeo respectively. Tang installed the Andong Protectorate after Goguryeo's fall and even built the Ungjin Areal Command in the old territory of Baekje. By doing so Tang intended to establish a world order centering on Tang China. However, Shilla was very much frustrated by Tang's actions and supported the revival movement of the Goguryeo refugees.
The countermeasures of not only Baekje and Goguryeo refugees but also Shilla against Tang's eastern policy can be summarized as follow. The revival movement of Baekje refugees was facing serious difficulties as the common goal to collapse Goguryeo still remained between Shilla and Tang. Therefore, the refugees of Baekje had no choice but to revive the Baekje kingdom by joining the Ungjin Areal Command. Meanwhile, the revival movement of Goguryeo was developed differently. Goguryeo refugees' revival movement which proceeded in the south area of the Ab'rok-gang river had helps from Shilla. Shilla's support stimulated the Andong Protectorate, and the revival movement of Goguryeo in Liaodong area resulted in the foundation of Balhae.
These two historical events, the installation of the Ungjin Areal Command and the foundation of Balhae, were the results of different choices made by Baekje and Goguryeo refugees against the Chinese-centric world order; the former accepted it, the latter rejected it. On the contrary, Shilla took the position of rivalry, while taking advantage of Tang's political attitude. Shilla rationalized its occupation over the old territory of Baekje by claiming that the world order with Tang at the center would not be possible if Shilla and the Ungjin Areal Command were separated. And Shilla invested Anseung as the king of Goguryeo in the same context. Shilla was in a position to demand its share for the old territory of Goguryeo. Shilla's attitude was part of its strategies to save itself while accepting the world order centered upon Tang China.