Furthermore, after Yeon-gaesomun died, a power struggle broke out among his three sons over the succession of the throne and this internal dissension threw Koguryo into doom. Lost out in the power struggle, the first son, Namsaeng, surrendered to the Tang forces, and revealed highly classified information about Koguryo. Repeated wars against Tang weakened Koguryo's production base. In addition, Koguryo leadership was split within itself and betrayal and distrust spawned amid dictatorship. Unable to withstand the onslaught of allied forces of the Tang and Silla, Koguryo finally collapsed in A.D. 668.

The after-effects of Koguryo's downfall were profound. With the exit of Koguryo, which used to form an axis of multi-polar civilizations, the Tang-centered uni-polar civilization emerged in East Asia, which in turn came to develop as the East Asian international order.

After the fall of Koguryo, many Koguryo people tried to revive their country. And, after 30 years of trying, Balhae(Pohai) was founded in 698 by Daejoyoung, a descendant of Koguryo. In official documents the king sent abroad, Balhae (698-926) used to boast itself as the successor of Koryo(Koguryo). (In the latter half of Koguryo Kingdom, people used to call their country "Koryo(Goryeo).") And, the kingdom of Koryo (or, Korea) (918-1392), which succeeded Balhae, resurrected the name of "Koryo," which was the state title of Koguryo.