Battle of Gangdong Fortress

When Khitan remnants fleeing the Mongols entrenched themselves at Gangdong, Goryeo attacked them together with Mongol and Eastern Xia troops — Goryeo’s first contact with the Mongols.

Khitan remnants driven out by the Mongols began crossing the Yalu in 1216 and ravaged Goryeo. Checked by Kim Chwi-ryeo and others, they shut themselves up in Gangdong Fortress east of Pyongyang in 1218. In the twelfth month of that year a Mongol army arrived in pursuit with Eastern Xia troops, and Goryeo sent Jo Chung and Kim Chwi-ryeo to join the siege. The Khitans surrendered in the first month of the following year.

Afterwards the Mongols imposed a "brotherly" alliance and demanded heavy annual tribute. When the Mongol envoy Zhuguyu was killed near the Yalu in 1225 on his way home with tribute, the Mongols used it as a pretext to invade in 1231.