The Three Kingdoms of Korea consisted of Goguryeo, Silla, and Baekje. Silla and Baekje controlled the southern half of the Korean Peninsula, maintaining the former Samhan territories, while Goguryeo controlled the northern half of the Korean Peninsula, Manchuria and the Liaodong Peninsula, uniting Buyeo, Okjeo, Dongye, and other states in the former Gojoseon territories.
Goguryeo was a highly militaristic state; it was a powerful empire and one of the great powers in East Asia, reaching its zenith in the 5th century when its territories expanded to encompass most of Manchuria to the north, parts of Inner Mongolia to the west, parts of Russia to the east, and the Seoul region to the south. Goguryeo experienced a golden age under Gwanggaeto the Great and his son Jangsu, who both subdued Baekje and Silla during their times, achieving a brief unification of the Three Kingdoms of Korea and becoming the most dominant power on the Korean Peninsula. In addition to contesting for control of the Korean Peninsula, Goguryeo had many military conflicts with various Chinese dynasties, most notably the Goguryeo-Sui War, in which Goguryeo defeated a huge force said to number over a million men. In 642, the powerful general Yeon Gaesomun led a coup and gained complete control over Goguryeo. In response, Emperor Tang Taizong of China led a campaign against Goguryeo, but was defeated and retreated. After the death of Tang Taizong, his son Emperor Tang Gaozong allied with the Korean kingdom of Silla and invaded Goguryeo again, but was unable to overcome Goguryeo’s stalwart defenses and was defeated in 662. However, Yeon Gaesomun died of a natural cause in 666 and Goguryeo was thrown into chaos and weakened by a succession struggle among his sons and younger brother, with his eldest son defecting to Tang and his younger brother defecting to Silla. The Tang-Silla alliance finally conquered Goguryeo in 668. After the collapse of Goguryeo, Tang and Silla ended their alliance and fought over control of the Korean Peninsula. Silla succeeded in gaining control over most of the Korean Peninsula, while Tang gained control over Goguryeo’s northern territories. However, 30 years after the fall of Goguryeo, a Goguryeo general by the name of Dae Joyeong founded the Korean-Mohe state of Balhae and successfully expelled the Tang presence from much of the former Goguryeo territories.