In the 10th century, Song in China, Kitan in Manchuria, and Goryeo in the Korean Peninsula were unified. However, territorial disputes frequently occurred as the three countries faced each other with borders centered on Kitan. In Liaodong, Kitan and Goryeo fought wars and diplomatic battles boringly over the Yalu River basin, which was Balhae‘s territory. With the accession of the sixth emperor(聖宗), his Eastern policy and Goryeo's northern policy clashed. Eventually, the Kitan invaded Goryeo, and with this opportunity, the two countries signed an agreement to make the southern part of the Yalu River a territory of Goryeo and the northern part a territory of Kitan. However, not only did he abandon his diplomatic cause and maintain effective control over the already ceded Boju(保州), but he also invaded again to return all six Gangdong states(江東六州). Boju is an area located in the lower reaches of the Yalu River and corresponds to the throat connecting Liaodong and the Korean Peninsula. Goryeo could not yield because it was a military hub and an important transportation and trade hub.
Goryeo protested several times to destroy various facilities of Kitan built in Boju and further asked for the return of the site, but failed to achieve its intention. Goryeo had to win Boju on a strategic level, but it was not able to forcibly recapture Boju due to its weak military power. In such a situation, Jurchen rose and founded the Jin Dynasty, and due to the fierce offensive of the Jin, Kitan was in danger, so Goryeo had a chance to acquire Boju again. Goryeo used diplomatic means rather than physical means to compromise with Kitan and Jin. When Kitan withdrew his troops, delivered Naewonseong Fortress(來遠城) and Boju to Goryeo. Goryeo occupied Boju before the Jin's army and asked the Jin Dynasty to recognize their ownership. Jin Dynasty recognized the ownership of Goryeo in return for subservience to the stronger. Finally incorporated Boju into Goryeo's territory.
In this way, Goryeo obtained six weeks of Gangdong and Boju by serve Kitan and Yeojin. Subservience to the stronger, in which a small country serves a large country, were international practices at the time. As long as they became a hegemonic power in Northeast Asia, it was a practical option for Goryeo. Goryeo acquired territory and maintained friendly relations between the two countries to minimize the war, which created favorable conditions for the development of Goryeo.