Yoo-Sin felt that Baekje, Goguryeo and Silla should not be separate countries, but should instead be united because all the people had the same ethnic background. He is regarded as the driving force in the unification of the Korean peninsula, and is the most famous of all the generals in the unification wars of the Three Kingdoms, but his victories were tempered by his regret that they had to be at the expense of people he considered to be ethnically the same as him.
His first military engagement is believed to have occured around 629, and through it he quickly proved his capabilities as a warrior. Silla was in a constant struggle with its neighbor to the west, Baekje, over territory. There had been gains and losses on both sides, and the struggle lasted for many years. It was during this period that Kim rose through the ranks of the military, rising to the position of general and becoming a skilled field commander.
Many stories exist about General Kim, Yoo Sin. It is told that once he was ordered to subdue a rebel army, but his troops refused to fight as they had seen a large shooting star fall from the sky and believed it to be a bad omen. To regain control, the General used a large kite to carry a fire ball into the sky. The soldiers, seeing the star return to heaven, rallied and defeated the rebels. It is also said that General Kim ingeniously used kites as a means of communication between his troops when split between islands and the mainland. It is also said that, another time, while Silla was allied with China against Baekje, an argument broke out between Yoo-Sin's commander and a Chinese general. As the argument escalated into a potentially bloody confrontation, Yoo-Sin's sword was said to have leaped from its scabbard into his hand. Because the sword of a warrior was believed to be his soul, this occurrence so frightened the Chinese general that he immediately apologised to the Silla officers. Incidents such as this kept the Chinese in awe of the Hwarang, and meant that in later years, when asked by the Chinese emperor to attack Silla, the Chinese generals refused, claiming that although Silla was small, it could not be defeated.
When Goguryeo and Baekje attacked Silla in 655, Silla joined forces with Tang Dynasty China to battle the invaders. Although it is not clear when Kim first became a general, he was certainly commanding the Silla forces at this time. Eventually, with the help of the Silla navy and some 13,000 Tang forces, Kim attacked the Baekje capital, Puyo, in 660, in one of the most famous battles of the century. The Baekje defenders were commanded by none other than General Ge-Baek, although the Baekje forces consisted of about 5,000 men and were no match for Kim's warriors, which numbered about ten times as many. Baekje, who had been experiencing internal political problems, crumbled. Kim's Silla forces and their Tang allies now moved on Goguryeo from two directions, and in 661 they attacked the seemingly impregnable Goguryeo Kingdom, but were repelled. The attack had weakened Goguryeo, though, and in 667 another offensive was launched which, in 668, destroyed Goguryeo forever. Silla still had to subdue various pockets of resistance, but their efforts were then focused on ensuring that their Chinese allies did not overstay their welcome on the peninsula. After some difficult conflicts, Silla eventually forced out the Tang and united the peninsula under their rule.
Kim was rewarded handsomely for his efforts in these campaigns. He reportedly received a village of over 500 households, and in 669 was given some 142 separate horse farms, spread throughout the kingdom. He died four years later at the age of 78, leaving behind ten children.
General Kim, Yoo-Sin lived to the age of 78 and is considered to be one of Korea's most famous generals of all time. Following his death in 673, General Kim was posthumously awarded the honorary title of King Heungmu, and was buried at the foot of Mt. Songhwasan, near Kyongju on the southeast coast of Korea, in a tomb as splendid as that of kings. He is the focus of numerous stories and legends, and is familiar to most Koreans from a very early age.