Yejong of Goryeo (11 February 1079 – 15 May 1122) (r. 1105–1122) was the 16th monarch of the Korean Goryeo dynasty. He was the eldest son of King Sukjong and Queen Myeongui, and succeeded to the throne upon his father's death. His reign is usually described as one of the most splendid periods of Goryeo, during which the arts and philosophy flourished, and military strengthening policies were implemented to ensure border security. Early years Yejong was born Wang U in 1079, the thirty-third year of his grandfather King Munjong's reign. He was the eldest son of King Sukjong, who took the throne in 1095 after a coup against King Heonjong, and of Queen Myeongui. He was made crown prince in 1100, aged 21, and became king in 1105 when Sukjong died returning from Jangnak Palace in Seogyeong. Foreign relations As soon as he ascended the throne, Yejong found himself having to face the conflict with the Jurchens that broke out during his father's reign. By the end of the eleventh century, the Wanyan tribe had begun to gain more power, leading to a fluctuation in the balances in Northeast Asia, and increasingly unstable relations with Goryeo, although they were initially part of Balhae in Manchuria and northern Korea's territories, and traced their descent from Goguryeo, referring to Goryeo as "father and mother country" and having particularly close contacts with its court. During Sukjong's reign, Goryeo had faced the Jurchens in two battles that ended in defeats. The main cause was identified in the composition of the Korean army, whose strength was the infantry, against the enemy's cavalry; it was therefore reorganized into the Byeolmuban army () to prepare for a large-scale war, which, however, did not take place due to the death of Sukjong. Goryeo and Jurchen maintained peaceful relations, but, after the period of mourning, Yejong decided to continue his father's work: when, in October 1107, the news that the Jurchens had invaded the North-Eastern border arrived to the court, an army of men was mobilized, led by General Yun Gwan (1040–1111). Within two months, Yun occupied the Hamhŭng plains and advanced into the Tumen basin; in April 1108, they completed the construction of nine forts to control the area, and measures were implemented to prevent the Jurchens from moving into the territory and finding supplies, but, due to incorrect geographic information, the Goryeo army found itself trapped in