[2] Traditional Korean histories from Joseon painted Yeon Gaesomun as a despotic leader, whose cruel policies and disobedience to his monarch led to the fall of Goguryeo.

[7] Since both Yeon Gaesomun and Iri Kasumi are transcriptions intending to approximate the phonetic value of the original pronunciation, his actual name can be reconstructed as "Eol Kasum".

[8] Very little is known of Yeon Gaesomun's early days, until he became the Western Governor (西部大人), where he oversaw the building of the Cheolli Jangseong, a network of military garrisons to defend Liaodong from Tang.

When Yeon Gaesomun discovered the plot, he arranged a lavish banquet to celebrate his rise to the position of Eastern Governor (東部大人) to which one hundred of the opposing politicians of the nation were invited.

Traditional Chinese and Korean historians assumed that Yeon Gaseomun's motive was simply his thirst for power, but many modern Korean historians assert that his motive was to make Goguryeo assume a tougher stance against Tang China, as opposed to King Yeongnyu who submitted to Tang for a peaceful diplomatic relationship.

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.

In 645, the first conflict of the Goguryeo–Tang War began and Emperor Taizong's noted military acumen enabled him to conquer a number of major Goguryeo border fortresses.

First, his main army was stymied and bogged down for several months at Ansi Fortress due to the resistance of the celebrated commander Yang Manchun.

[20][21] Famed Tang general Su Dingfang, who was instrumental in conquering Baekje, was unable to overcome Pyongyang's defenses and was forced to withdraw due to harsh snowstorms.

[24][25] Tang and Silla sources portrayed Yeon Gaesomun as a brutal and arrogant dictator who carried five swords at a time, and had men prostrate themselves so that he could use their backs to mount and dismount his horse.

[29] Age of Empires: World Domination, a mobile game produced in collaboration with series owner Microsoft,[30] includes Yeon Gaesomun as a selectable hero of the Korean civilization.