The Silla–Tang War that lasted from 668 CE to 676 CE happened between the Korean Silla state and Goguryeo and Baekje’s remnant forces and the mighty Chinese Tang empire. It started in the geopolitical context immediately after the conquest of Baekje and Goguryeo by Tang and Silla.
In 668 CE, Silla failed to consolidate the entire Korean Kingdoms after the fall of Goguryeo. People of Goguryeo came together to form a new kingdom. Silla would declare war on Tang and push Tang back to their border. Before the Silla-Tang alliance, Tang was ravaged by defeats suffered under the Goguryeo military. These defeats pushed Tang to form an alliance with Silla, which was under the Goguryeo domain. The Silla–Tang alliance, which had beaten both the Korean kingdoms of Goguryeo and Baekje, resulted in the occupation of the former colonies of both of these kingdoms by coalition (i.e., Tang and Silla) armies.
Silla’s King Munmu gathered the Silla men with the armies of the defeated kingdoms Baekje and Goguryeo in an alliance against the Tang army. He had the hard challenge of saving his nation and hard-won conquests (especially the entirety of what was once Baekje and some of the southern portions of what was once Goguryeo) from Tang Dynasty domination. To accomplish this, Munmu created alliances with Goguryeo resistance leaders such as Anseung and Geom Mojam, and propelled attacks on the Tang army holding territories formerly belonging to the defeated kingdoms of Goguryeo and Baekje. Starting almost immediately after the collapse of Goguryeo in 668 CE and the institution of the office of Protectorate General to Calm the East, the conflict between the ancient allies lasted through much of the 670s CE.
By 674 CE, Silla and Tang were engaged in frequent battles, as King Munmu had sized over much of the former territories of Goguryeo and Baekje from the Tang forces that had conquered these lands following the Baekje–Tang War and Goguryeo–Tang War. They promoted native resistance against the Tang army in those regions of the other Korean kingdoms still owned by the Tang. In that very year, Emperor Gaozong, in anger, randomly declared King Munmu’s brother Kim Inmun to be the king of Silla and ordered Liu Rengui with an army to invade Silla. However, before they could fight any major battles, King Munmu, for unexplained reasons (probably unwilling to deal with the possible divisions that could be produced by having a royal pretender lose in his kingdom and inexperienced to fight the Tan, formally retracted and presented tribute; this satisfied Emperor Gaozong, and, not wanting to prolong the campaign, he ordered a retreat and recalled Kim Inmun.
Afterward, in 676 CE, the Tang administrations moved the Protectorate General to Cam the East to Liaodong. Although the Tang troops lost the captured territories south of the Taedong River, Silla could not retrieve the former Goguryeo territories to the north of the Taedong River, which had fallen into the Tang realm. The Tang empire had seized control of the Liaodong Peninsula, while Silla dominated the Korean Peninsula.
Ultimately, Emperor Xuanzong of Tang (712 CE–755 CE) reestablished diplomatic ties and started a reconciliation between the states.
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