Military history of Goguryeo

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.

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 Kyushu in the Japanese archipelago.

Gogukyang, a son of Gogukwon, invaded Baekje in 386.

...
Wikipedia