How a brutal internal power struggle collapsed the mighty Goguryeo kingdom in 668
For decades, Goguryeo stood as a formidable barrier against the Tang dynasty's expansion. Yet, after the death of its military dictator in 666, the kingdom fractured from within. This internal collapse allowed the Tang–Silla alliance to dismantle the state, ending the Three Kingdoms period of Korea forever.
The Goguryeo–Tang War, spanning 645 to 668, was defined by shifting alliances and grueling sieges. Initially, Goguryeo successfully repelled Tang invasions between 645 and 648. Emperor Taizong’s 645 campaign, which included a massive force of 60,000 soldiers and a fleet of 500 ships, famously stalled at Ansi Fortress. Despite the Tang army’s use of incendiary projectiles and the construction of a massive siege mound, the defenders under Yang Manch'un held firm, forcing a difficult Tang withdrawal in October 645.
The tide turned following the death of the Goguryeo dictator Yŏn Kaesomun in 666. His passing triggered a violent succession struggle among his sons and brother, leading to widespread demoralization and critical defections. Yŏn Kaesomun’s eldest son, Yŏn Namsaeng, defected to the Tang, providing the alliance with invaluable intelligence and support. This internal chaos crippled Goguryeo’s ability to coordinate a defense, leaving the kingdom vulnerable to a coordinated pincer movement from the north and south by Tang and Silla forces.
By late 668, the exhausted Goguryeo state succumbed. Tang and Silla forces conquered Pyongyang on 22 October 668, taking over 200,000 prisoners. The Tang Empire subsequently established the Protectorate General to Pacify the East to govern the former territories. However, this victory was short-lived as a source of stability. The logistical strain of occupation and the Tang Empire's shifting priorities soon led to the Silla–Tang War, as Silla moved to resist Chinese dominion over the peninsula.
Source: Goguryeo–Tang War
