清川江
Following the "Manbu Bridge Incident" of 942, Goryeo prepared itself for a conflict with the Khitan Empire: Jeongjong established a military reserve force of 300,000 soldiers called the "Resplendent Army" in 947, and Gwangjong built fortresses north of the Chongchon River, expanding toward the Yalu River. The Khitans considered Goryeo a potential threat and, with tensions rising, invaded in 993. The Koreans were defeated in their first encounter with the Khitans, but successfully defended against them at the Chongchon River. Negotiations began between the Goryeo commander, Seo Hui, and the Liao commander, Xiao Sunning. In conclusion, Goryeo entered a nominal tributary relationship with Liao, severing relations with Song, and Liao conceded the land east of the Yalu River to Goryeo. Afterward, Goryeo established the "Six Garrison Settlements East of the River" in its new territory. In 994, Goryeo proposed to Song a joint military attack on Liao, but was declined; previously, in 985, when Song had proposed a joint military attack on Liao, Goryeo had declined. For a time, Goryeo and Liao enjoyed an amicable relationship. In 996, Seongjong married a Liao princess.