[4][5] In regard to the Korean Peninsula, Gwanggaeto defeated Baekje, the then most powerful of the Three Kingdoms of Korea,[4] in 396, capturing the capital city of Wiryeseong in present-day Seoul.

[15] In 399, Silla, the southeastern kingdom of Korea, sought aid from Goguryeo due to incursions by Baekje troops and their Wa allies from the Japanese archipelago.

In 371, three years prior to Gwanggaeto's birth, the rival Korean kingdom of Baekje, under the great leadership of Geunchogo, soundly defeated Goguryeo, slaying the monarch Gogukwon and sacking Pyongyang.

Baekje's influence was not limited to the Korean peninsula, but extended across the sea to Liaoxi and Shandong in China, taking advantage of the weakened state of Former Qin, and Kyushu in the Japanese archipelago.

[26] Goguryeo was inclined to avoid conflicts with its ominous neighbor,[27] while cultivating constructive relations with the Former Qin,[28] the Xianbei, and the Rouran, in order to defend itself from future invasions and to bide time to reshape its legal structure and to initiate military reforms.

[34][39] In 395, while his campaign against Baekje was ongoing to the south, Gwanggaeto made an excursion to invade the Khitan Baili clan to the west on the Liao River,[40] destroying 3 tribes and 600 to 700 camps.

[42] In 400, while Gwanggaeto was occupied with Baekje, Gaya, and Wa troops in Silla, the Xianbei state of Later Yan, founded by the Murong clan in present-day Liaoning, attacked Goguryeo.

In 404, Gwanggaeto defeated an attack by the Wa from the Japanese archipelago on the southern border of what was once the Daifang commandery, inflicting enormous casualties on the enemy.

[50] Gwanggaeto's conquests are said to mark the zenith of Korean history, building and consolidating a great empire in Northeast Asia and uniting the Three Kingdoms of Korea under his influence.

[2][5] Except for the period of 200 years beginning with Jangsu, who would build upon his father's domain, and the golden age of Balhae, Korea never before or since ruled such a vast territory.

[51][52][53] Gwanggaeto established his own era name, Yeongnak Eternal Rejoicing, proclaiming Goguryeo monarchs equal to their counterparts in the Chinese mainland.

The Republic of Korea Navy operates Gwanggaeto the Great-class destroyers, built by Daewoo Heavy Industries and named in honor of the monarch.

[65] Age of Empires: World Domination, a mobile game produced in collaboration with series owner Microsoft,[66] includes Gwanggaeto as a selectable hero of the Korean civilization.