King Gogukwon’s successors were Kings, Sosurim and Gogukyang. They began to overhaul the state’s internal institutions in a bid to overcome a series of crises facing Koguryo. Kings Sosurim and Gogukyang were able to solidify state power by firmly establishing social discipline through administrative laws and orders, providing ground for liberal thoughts by introducing Buddhism, and nurturing intellectuals with the creation of Taehak (National Academy).
It was King Gwanggaeto (r. 391-413), who greatly changed the map of Koguryo by vigorously pursuing the task of adding new domains by conquest. The great military campaigns of this king made Silla in the southeastern part of the Korean peninsula a vassal state, and Baekje, Gaya and Japan its tributary states. To the north, it overpowered Suksin and East Buyeo and made them tributary states. To the west, Koguryo conquered the Khitan tribe along the middle and upper reaches of West Liao River and destroyed the Later Yen, set up by Moyong Seonbi tribe.

King Gwanggaeto was succeeded by his son, King Jangsu. During his 79 years on the throne, King Jangsu continued his father’s enterprises and brought Koguryo to its flourishing height. Koguryo’s power expanded further to the north, dividing the Jiduwu territory along the Greater Khingan Range with the nomadic empire of Yuyen. To the south, Koguryo exercised direct control over a region between the Gyeonggi Bay and the Youngil Bay in North Gyeongsang Province (the mid-section of Korean Peninsula). Subsequently, Koguryo continued to expand its territory until it reached the Eastern Songhua River in the north, Mt. Uimuryeo across Liao River in the west and the southern part of today’s Littoral Province of Russia. If its indirect control over nomadic tribes is taken into consideration in addition to the newly added territories, Koguryo at that time was a great power with its sphere of influence covering a large amount of present-day Northeast China, Russia’s Littoral Province and two thirds of the Korean peninsula. Inside the huge sphere of influence were Khitan, Malgal (Moho), Jiduwu, Silla and Ye in the form of vassal or tributary states. Koguryo at this time was a great empire that took pride as the guardian of international order and the most sacred state under the sun.

Koguryo, in the fifth and sixth centuries, was one of the four great powers in East Asia, along with Yuyen, a nomadic empire; Northern Wei, a state set up by the Seonbi tribe in the Huang River basin, and Song, a state established by Han China in the Yangtze River basin. Unlike Northern Wei and Song, which were persistently buffeted by wars and other disturbances, Koguryo was able to nurture a high quality culture, while enjoying peace. Indeed, Koguryo culture exercised a significant impact on Baekje, Silla, and Japan.

In early sixth century, Northern Wei, ruler of North China, likened Koguryo to a large wine barrel, and recognized the world of Koguryo’s independent dominion east of the Great Wall.

[Source : Korea.Net]

Even in its heyday, a crisis once befell Koguryo. There was a series of confrontations among the nobility over the succession issue in the 540s. Taking advantage of this domestic disarray, the newly emerging nomadic power, Tuchueh, launched an attack from the north in the 550s and Baekje and Silla attacked from the south. Koguryo in the end had to cede the mid-section of the Korean peninsula to Silla and concentrated its efforts on repulsing Tuchueh, and was victorious.
A more serious challenge descended upon Koguryo during the second half of the sixth century. The four-power set-up in East Asia was collapsing, when the new Sui Empire emerged. Sui managed to unify numerous nations competing for supremacy in the Huang and Yangtze River regions and even destroyed Tuchueh, who had been ruling the northeastern plains. This led Sui forces face to face with Koguryo, which was still pursuing expansionist policies. In preparation for a war with Sui, Koguryo collected lots of information, spurred on weapons development, strengthened its internal cohesion and braced itself for a war. In 612, Sui attacked Koguryo with millions of troops. Koguryo scored a major victory at the battle of Salsu River by annihilating most of the 305,000-strong Sui troops. Subsequently, Koguryo successfully repulsed three more major attacks. Ultimately, this series of defeats led to the downfall of the Sui Empire itself.

Thirty years later, Koguryo had to wage a war against the Tang, the Chinese successor to Sui. In 645, it repulsed hundreds of thousands of Tang forces in combats at the western frontier’s strategic points of Sinseong, Geonan and Anshi fortresses and Mt. Jupil. The commander of Chinese forces was King Taejong himself, known as the greatest king in Chinese history. In 661, mobilizing hundreds of thousands of troops, Tang again came attacking Koguryo, but its ambitions were frustrated the following February as the main component of Chinese forces was routed.

[Source : Korea.Net]

All 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. 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.

[Source : Korea.Net]