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.