The military history of Goguryeo involves wars with other Korean kingdoms, Chinese dynasties, nomadic states and tribes, and Wa Japan. Goguryeo was a highly militaristic state; it was a powerful empire and one of the great powers in East Asia, until it was defeated by a Silla–Tang alliance in 668 after prolonged exhaustion and internal strife caused by the death of Yeon Gaesomun.
Conflicts with other Korean states
Baekje
Goguryeo and Baekje were two of the Three Kingdoms of Korea; both claimed descent from the ancient Korean kingdom of Buyeo. Onjo, the founder of Baekje, was said to be the second son of Jumong, the founder of Goguryeo. Despite the common ancestry, the relationship between Goguryeo and Baekje was often contentious.
During the 4th century, Geunchogo expanded Baekje's territory to the north at the expense of Goguryeo. In 369, Gogukwon, the monarch of Goguryeo, attacked Baekje with 20,000 troops, but was defeated by Crown Prince Geungusu at the Battle of Chiyang. In 371, Geungusu led 30,000 troops and attacked the fortress of Pyongyang, slaying Gogukwon in battle. Baekje was a powerful maritime nation whose influence extended across the sea to Liaoxi and Shandong in China, taking advantage of the weakened state of Former Qin, and to Kyushu in the Japanese archipelago.
Gogukyang, a son of Gogukwon, invaded Baekje in 386.
In 392, Gwanggaeto the Great led an attack on Baekje with 40,000 troops, capturing 10 walled cities. In response, Asin, the monarch of Baekje, launched a counterattack on Goguryeo in 393 but was defeated. Asin invaded Goguryeo once more in 394, but was defeated again. After suffering multiple defeats against Goguryeo, Baekje's political stability began to crumble. In 395, Baekje was defeated once more by Goguryeo and was pushed south to its capital of Wiryeseong on the Han River. In the following year, in 396, Gwanggaeto led an assault on Wiryeseong by land and sea, using the Han River, and triumphed over Baekje. Gwanggaeto captured the Baekje capital and the defeated Asin submitted to him, surrendering a prince and 10 government ministers. Henceforth, Baekje lost its dominance in the central Korean peninsula and declined.
In 400, Silla requested aid from Goguryeo in repelling an allied invasion by Baekje, Gaya, and Wa. Gwanggaeto dispatched 50,000 troops and annihilated the enemy coalition.
In 433, Baekje and Silla formed an alliance (Hangul: 나제동맹, Hanja: 羅濟同盟) to balance the Goguryeo threat to the north.
In 472, Gaero, the ruler of Baekje, requested a military alliance with Northern Wei against Goguryeo, but was unsuccessful. In 475, Jangsu, the son of Gwanggaeto, launched an invasion by land and sea against Baekje, and captured the capital of Wiryeseong and executed Gaero. Baekje had no choice but to move its capital to mountainous Ungjin (present-day Gongju), 80 miles to the south, which provided a natural protection for the devastated kingdom.
In 479, Baekje and Silla reaffirmed their alliance through marriage, which was the primary reason why Goguryeo was unable to conquer the entire peninsula.
In 551, a Baekje–Silla alliance attacked Goguryeo in order to capture the important Han River region from Goguryeo, planning to split it between them. In 553, Baekje gained the critical region after expending itself with a series of costly assaults on Goguryeo fortresses, but Silla troops, arriving on the pretense of offering assistance, attacked the exhausted Baekje troops and took possession of the entire Han River region, leading to a war between the two former allies in which the Baekje monarch was killed.
Baekje attacked Silla in 612, 624, and 627. Goguryeo and Baekje formed an alliance (Hangul: 여제동맹, Hanja: 麗濟同盟) in 642 aimed toward territorial restoration against Silla. King Uija of Baekje attacked Silla and captured around 40 strongpoints in 642, and 7 more fortresses in 645. In 655, the Goguryeo–Baekje alliance captured 30 fortresses from Silla.
Silla
In 245, Dongcheon ordered an attack on Silla, but the two kingdoms entered into friendly relations in 248.
Nulji, the king of Silla, who had been a vassal of Jangsu, broke off relations with Goguryeo in 454. Jangsu invaded Silla in 468, expanding his domain into parts of Gangwon Province, and again in 489, capturing 7 walled cities and expanding his domain into parts of North Gyeongsang Province.