Battle of Gwiju
Goryeo forces under Gang Gam-chan crushed a 100,000-strong Khitan army at Gwiju.
Dreamline91 · CC BY-SA 4.0 · Source
When Xiao Paiya invaded with 100,000 men in the twelfth month of 1018 (the third Khitan invasion), Gang Gam-chan as commander-in-chief and Gang Min-cheom as his deputy met him with some 208,000 troops. At Heunghwajin Gang Gam-chan dammed a stream with strung cowhides, released it as the enemy crossed and fell on them with hidden cavalry. Xiao Paiya marched straight on Gaegyeong, but Goryeo emptied the countryside and held the walls, and he turned back near the capital.
In the second month of 1019 Gang Gam-chan met the retreating Khitans on the plain at Gwiju. At the height of the battle reinforcements under Kim Jong-hyeon arrived and wind and rain blew from the south into the Khitans’ faces, and Goryeo won a crushing victory. The Goryeosa records that only a few thousand Khitans made it home.
The victory ended the wars with the Khitans that had begun in 993. Goryeo made peace while keeping the six garrisons east of the Yalu, and a balance of power settled among Goryeo, Song and Liao. At Gang Gam-chan’s urging an outer wall was built around Gaegyeong (1029), and the Cheolli Jangseong was raised along the northern frontier.