The Goguryeo–Tang War (645–668) was a conflict between the Goguryeo kingdom and the Tang Dynasty, marked by alliances with various states and military strategies. The initial phase of the war (645–648) saw Goguryeo successfully repelling the Tang forces. However, after Tang and Silla's joint conquest of Baekje in 660, they launched a coordinated invasion of Goguryeo in 661, only to be forced to retreat in 662. The death of Goguryeo's military dictator, Yeon Gaesomun, in 666 led to internal strife, defections, and demoralization, which played into the hands of the Tang–Silla alliance. They launched a renewed invasion in 667, and by late 668, Goguryeo succumbed to the numerically superior armies of the Tang Dynasty and Silla, marking the end of the Three Kingdoms of Korea period and setting the stage for the subsequent Silla–Tang War.^[56]

The war's onset was influenced by Silla's requests for Tang's military support against Goguryeo and their concurrent conflict with Baekje. In 641 and 642, the Goguryeo and Baekje kingdoms saw power shifts with the rise of Yeon Gaesomun and King Uija, respectively, leading to increased hostilities and a mutual alliance against Tang and Silla. Emperor Taizong of Tang initiated the first conflict in 645, deploying a substantial army and fleet, capturing several Goguryeo strongholds, but ultimately failing to take the Ansi Fortress, resulting in a Tang retreat.^[57]

In the subsequent phases of the war (654–668), under Emperor Gaozong, the Tang Dynasty formed a military alliance with Silla. Despite initial setbacks and a failed invasion in 658, the Tang-Silla alliance successfully conquered Baekje in 660. The focus then shifted to Goguryeo, with a failed invasion in 661 and a renewed attack in 667 following Yeon Gaesomun's death and the resulting Goguryeo instability. The war concluded with the fall of Pyongyang and the conquest of Goguryeo in 668, leading to the establishment of the Protectorate General to Pacify the East by the Tang Dynasty. However, logistical challenges and a strategic shift towards a more pacifist policy by Empress Wu, amidst Emperor Gaozong's failing health, ultimately set the stage for the resistance and the forthcoming conflict between Silla and Tang.^[58]