Traditional Korean histories paint Yeon as a despotic leader, whose cruel policies and disobedience to his monarch led to the fall of Goguryeo. However, his achievements in defending Goguryeo against Chinese onslaughts have inspired Korean nationalist historians, most notably the 19th century Korean historian and intellectual Sin Chaeho, to term Yeon the greatest hero in Korean history. Many Korean scholars today echo Sin and praise Yeon as a soldier-statesman without equal in Korean history, though other scholars strongly disagree. Chinese and Japanese scholars continue to hold an unfavorable view of Yeon.
Tang Chinese historical records give Yeon Gaesomun's surname as Cheon 泉 (Chinese, Quan, meaning "spring"), because Yeon (Chinese 淵, Yuan, meaning "riverhead") was the given name of Emperor Gaozu of Tang (Li Yuan, 李淵), founder and first emperor of Tang, and thus taboo to apply to another by Chinese tradition (see naming taboo). He is also sometimes referred to as Gaegeum (개금/蓋金). In Nihon Shoki, he appears as Iri Kasumi (伊梨柯須彌).
Very little is known of Yeon's early days, until he became the Governor of the Western province (西部), where he oversaw the building of the Cheolli Jangseong, a network of military garrisons to defend the Liaodong area from Tang invaders.
In 642, Yeon arranged a lavish banquet to celebrate his rise to the position of Eastern Governor to which one hundred of the opposing politicians of the kingdom were invited. Yeon's soldiers ambushed and killed all one hundred ministers present. Yeon then proceeded to the palace and murdered the king. According to traditional Chinese and Korean sources, Yeon's men dismembered the dead king's corpse and discarded it without proper ceremony. After placing Bojang Taewang (r. 642-668),a nephew of Yeongnyu, on the Goguryeo throne, Yeon appointed himself Dae Mangniji (대막리지, 大莫離支, the highest possible rank of Goguryeo under the Taewang; associated with commander of military affairs, and leader of political affairs). Subsequently, in this role Yeon went on to assume de facto control over Goguryeo affairs of state until his death around 666.
His role in the murder of the Goguryeo king was taken as the primary pretext for the failed Tang invasion of 645.
At the outset of his rule, Yeon took a briefly conciliatory stance toward Tang China. For instance, he supported Taoism at the expense of Buddhism, and to this effect in 643 sent emissaries to the Tang court requesting Taoist sages, eight of whom were brought to Goguryeo. This gesture is considered by some historians as an effort to pacify the Tang and buy time to prepare for the Tang invasion Yeon thought inevitable given his ambitions to annex Silla.
Relations with Tang deteriorated when Goguryeo launched new invasions of Silla. In 645 the Goguryeo-Tang Wars began and Taizong's noted military acumen enabled him to conquer a number of major Goguryeo border fortresses.
Eventually, however, Taizong's invasion was met with two major setbacks. First, Taizong's main army was stymied and bogged down for several months at Ansi Fortress due to the resistance of the celebrated commander, Yang Man-chun. Second, the elite marine force that Taizong sent to take Pyongyang, Goguryeo's capital, was defeated by Yeon who, according to the Joseon Sanggosa, then immediately marched his legions to relieve Yang's forces at Ansi Fortress.
Taizong, caught between Yang's army in the front and Yeon's counter-attacking forces closing in from behind, as well as suffering from the harsh winter and dangerously low food supplies, was forced to retreat homeward (Zizhi Tongjian). During the retreat itself, a large number of Taizong's soldiers were slain by Yeon and his pursuing army. However, Taizong and the invading army survived. Taizong's first invasion of Goguryeo was thus defeated. However, Taizong succeeded in inflicting heavy casualties on Goguryeo.
It is speculated that after Taizong's failure to conquer Goguryeo his personal rivalry with Yeon became an obsession with both him and his son Gaozong. They invaded Goguryeo two more times in 661 and 667 and were unsuccessful in both attempts--perhaps most notably during Yeon's victory over the Tang forces in 662 at the Salsu River (蛇水, Probably present-day Botong river) where the Tang commander Pang Xiaotai and his 13 sons were killed in battle. With increasing domestic turmoil in China, Tang was once again forced to retreat.
However, the population and economy were severely damaged after the three major invasions and never fully recovered. Silla and Tang continued their mutual struggle for over 8 years. In 668, two years after Yeon's death, Goguryeo finally met its demise. However, at least during the rule of Yeon Gaesomun the Tang-Silla alliance was unable to subdue Goguryeo.
He apparently died of natural causes.
Yeon Gaesomun had at least three sons, (eldest to youngest) Yeon Namsaeng, Yeon Namgeon, and Yeon Namsan. After his death, the country was weakened by a succession struggle between his brother and three sons, and in 668 fell relatively swiftly to the Silla-Tang armies.
First, in terms of his character, later Confucian scholars have mercilessly criticized Yeon for the coup and the regicide that brought him to power. In their eyes, he was a disloyal subject who sought personal power above all else. In particular, extant Tang and Silla sources have consistently portrayed Yeon as a brutal and arrogant dictator. According to their testimony, for instance, Yeon carried five swords at a time, and would have men prostrate themselves so that he might use their backs to mount or dismount his horse.
Yeon's modern-time defenders, however, dismiss these Tang and Silla sources as biased calumnies of enemy historians. Moreover, they argue that Yeon's subsequent single-mindedness and success in defending Goguryeo testifies his genuine patriotism (though Taizong's first invasion was provoked by Yeon's attacks on Silla, and subsequent attacks were possibly due to Taizong and his son's personal hatred against Yeon).
Second, in terms of his role in the downfall of Gogureyo, Yeon's detractors blame Yeon for needlessly provoking the Tang to attack Goguryeo (see above) and thereby ensuring its downfall. They point out that, while Goguryeo remained a formidable regional power before Yeon assumed power, it was completely destroyed by Silla and Tang within a short time soon after his death. They also point out that the population of Gogureyo decreased dramatically during Yeon's rule, and much of the economy was destroyed due to constant wars with Tang China and Silla.
Yeon's defenders rejoin by claiming that the Tang would have invaded Goguryeo, regardless of Goguryeo's attitude vis-a-vis Tang (although a major reason for Taizong's first invasion of Goguryeo was Yeon's invasion of Silla, another Korean kingdom that allied with Tang). They add that continuing to appease the Tang--as King Yongnyu had done--is tantamount to Neville Chamberlain's appeasement of Hitler.
For many modern Korean nationalists historian, Yeon is a symbol of that magic historical juncture where the pinnacle of Korean history and its Chinese counterpart violently collided, and the Koreans unambiguously triumphed over the Chinese. It is no wonder in this age of renascent conflict between South Korea and China over the historical ownership of part of Manchuria, Yeon has undegone a dramatic rehabilitation, and he is for the first time by a large number of South Koreans, most of them descendants of the people of Silla, the greatest hero in their history per Sin Chae-ho's words.
Another huge controversy that arises is the sources actually used to support the defeat of the Tang Dynasty. Some sources such as Sin's Joseon Sanggosa claim that Taizong was forced into the outskirts of Beijing. However Sin's account has been challenged on the basis that it lacked support in traditional Korean and Chinese sources. The ancient Korean history Samguk Sagi and ancient Chinese histories Book of Tang, New Book of Tang, and Zizhi Tongjian put the figure at 20,000, stating that there were only 100,000 Tang soldiers used total. The modern Chinese historian Bo Yang has speculated that the Yeon had the records altered so that he could claim credit for Yang Manchun's victory over Tang. However, Sin's have the possibility claimed that some suspect Chinese designation and Taizong's retreat course. therefore, modern Korean historian ponder upon that such a claim.