Chinese Emperor Gao Zong, the "High Ancestor," concluded an alliance with Silla not to assist Silla in its conflict with Paekche and Koguryo, but to bring the entire peninsula under imperial control of the Tang Dynasty. The alliance merely provided a convenient method for China to get its foot on the peninsula without having to fight everyone in sight. The Tang emperor ordered a force of thirty-five armies into Korea in the spring of 660 AD, intending to first defeat the Kingdom of Paekche in alliance with Silla, then turn north against the Kingdom of Koguryo. The large invasion fleet that crossed the Yellow Sea from China carried nearly 123,000 crack Chinese troops under the command of Left Tiger Guard General Su Dingfang, fresh from his victory over the Tujue three years earlier, and General Liu Boying. The Chinese sailed toward a rendezvous with Silla's large army, which had already established a major encampment at Namch'on .
The Chinese fleet dropped anchor near the mouth of the Kum River, and sent a messenger to the Silla camp to announce their arrival. King Muyol ordered Crown Prince Pommin and his senior officers, including General Kim Yu-sin, to meet with the Chinese to coordinate the impending assault on Paekche. General Su Dingfang told Crown Prince Pommin that Chinese forces would move into Paekche by the lower reaches of the Kum River while Silla moved in from the east, and "We will meet at the walls of Sabi, Paekche's capital, on the tenth of the seventh month." The Chinese force landed near Ibolp'o, laying willow rush matting across the wide expanse of thick coastal mud that blocked their approach. In the east, King Muyol and Crown Prince Pommin moved their army to a forward encampment at Sara.
Paekche's King Uija had long ignored his senior officials' advice regarding matters of defense. Now under severe pressure from a two-pronged assault against his kingdom, King Uija moved too little and too late. In sheer desperation, he ordered a small detachment of warriors to block Silla's advance from the east while he and his retinue fled north to Ungjin near modern Kongju to seek refuge.
Three large armies pushed eastward into the area around Hwangsan near modern Yonsan led by General P'umil, General Kim Humch'un and General Kim Yu-sin. General P'umil's son, Kwanch'ang, was a handsome sixteen year old whose horsemanship and archery skills brought him to the attention of King Muyol. Appointed an adjunct general under his father's command, Kwanch'ang rode into battle against General Kyebaek's troops on the plain at Hwangsan
The battle had not gone well for Silla during the early fighting, but General P'umil's actions deeply strengthened their resolve to win. Under the hot summer sun of early July 660 AD, three Silla armies beat their drums, shouted war cries, and charged headlong into the enemy lines. In a brief but violent engagement at Hwangsan near modern Yonsan, Silla warriors mauled the badly outnumbered Paekche soldiers. King Muyol conferred a posthumous social rank on Kwanch'ang, buried him with full rites, and sent thirty rolls each of Chinese silk and cotton and one hundred sacks of grain for his funeral expenses. Many of Silla's legendary hwarang warriors earned their reputations during the Tang-Silla campaign against Paekche.
The fighting had gone on several days longer than expected, and one of Silla's commanders arrived late for a meeting with the Chinese general. General Su Dingfang angrily wanted to punish the commander for his late arrival and disobedience and ordered the man's execution. General Kim Yu-sin learned of the execution order when he met with General Su to report the news of his victories. Suspicious that China's real intentions were to first take Paekche and then attack Silla, in a sudden burst of anger, General Kim ripped his sword from its scabbard and threatened to fight the Tang army first then defeat Paekche. The sudden outburst stunned the Chinese commander, who quickly apologized to General Kim and rescinded his execution order.
The Silla and Tang armies linked up shortly afterwards and swept unimpeded toward the capital at Sabi. Paekche forces made their last stand at the Pusosansong Fortress, located on a steep hill near Sabi. According to legend, some 3,000 women of the Paekche court committed mass suicide by leaping from Nak'waam Rock, the "Rock of Falling Flowers," into the Paengma River below rather than allowing themselves to be captured.
The uneasy alliance between Tang China and Silla held firm during the conquest of Paekche, but Silla spies among the Tang armies encamped on the hills surrounding Sabi soon learned of Chinese plans to invade Silla. King Muyol gathered his generals and ministers to develop a strategy for dealing with this potential disaster. Lord Tami suggested that Silla men dress as Paekche warriors and act as if they are going to rebel. The Tang army would certainly strike out at such a development and give Silla an excuse to attack the Chinese. General Kim Yu-sin agreed with the proposal and asked the king for approval to proceed. Chinese spies soon learned of the plan, however. General Su Dingfang offered all the conquered Paekche territory as maintenance lands to General Kim Yu-sin and two other high ranking Silla officers "as reward for your merit." General Kim refused the offer.
In early September, General Su Dingfang took King Uija, ninety-three ministers, and twenty thousand soldiers as prisoners, and set sail for China, leaving General Liu Renyuan in Paekche to command an occupation army. After presenting his prisoners, General Su received words of commendation and indebtedness from Emperor Gao Zong, who also asked why he had not followed through with an attack on Silla. The general replied that, although Silla was indeed a small kingdom, its king was a very wise man and its generals were fierce and loyal fighters. He repeated the incident with General Kim Yu-sin and told the emperor that China could not successfully plot against them.