Ch‘oe U followed in his father’s steps and having established himself in the viceroyship began to abuse the people, stealing houses and lands from them wherewith to build himself a princely mansion, two hundred paces long. In the court of it he had mock battles and the soldiers played at ball. The expense of this was borne by the people, whose faces were already being ground to furnish the regular revenue. His younger brother, Hyang, who long since been had banished, attempted to raise an insurrection in favor of the exiled king; but Ch‘oe U sent a strong force and chased his brother until he was run to earth in a cave among the mountains where he was killed. It was now the year 1231, the year which saw the outbreak which had been threatening ever since Genghis Khan came to the chieftainship of the Mongol armies. As the spring opened a powerful Mongol army moved southward across the Yalu under the leadership of Sal Ye-t‘ap and took the fortress of Ham-sin near Eui-ju. They followed this up by storming Ch‘ŭl-ju which ended only after the prefect had set fire to his house and destroyed his whole family and he and his associates had cut their own throats. The king did not intend to submit without a struggle. He sent Generals Pak Sö and Kim Kyöng-sol at the head of a large army to operate against the invaders. They rendezvoused with all their forces at Ku-ju, the four gates of which were strongly barricaded. The Mongols commenced the attack at the south gate. The Koryŭ soldiers made five brilliant sallies and forced the enemy to retire. The honors of this victory fell to Gen. Kim who pursued the enemy some distance and then returned to the town in triumph. The Mongols, who seem to have been independent of any base of supplies and made the country through which they passed supply them, now left this town untaken and the Koryŭ army undefeated in their rear, and marched boldly southward, taking Kwak-ju and Sŭn-ju. From this point the Mongol general Sal Ye-t‘ap sent a letter to the king saying “Let us make peace. We have now taken your country as far as Han-sin and if you do not come to terms with us we will draw reinforcements from Yŭ-jin and crush you.” The messenger who conveyed this very candid letter got only as far as P‘yŭng-ju where he was seized by the people and imprisoned. While waiting for an answer, the invaders tried another attack on Ku-ju but with no better success. Not only so, but they were badly defeated at Au-puk fortress.