great powers in history perished in the long run. And,
Koguryo was no exception. Yeon Gaesomun, who seized power
in 642, was a great general. He led Koguryo to victory
in wars against Tang in 645 and 662. But, he also left
the legacy of dictatorship. Dictatorship narrowed the
scope of political recruiting and rendered the state insensitive
to new international developments.
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.
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.
[Source : Korea.Net]