In 1261 the emperor made a requisition upon Koryŭ for a large amount of copper and lead. The king did not have the copper and yet did not dare to refuse; so he sent to A-t‘o in China and bought copper and delivered it as ordered, but told how he had procured it. The emperor charged him with lying and claimed that he was remiss in her duties as a vassal. He moreover ordered that the king take a census of Koryŭ, establish a horse relay system, train soldiers and prepare provisions for an army. The king was unable to comply and an estrangement grew up between him and the emperor which was unfortunate for both. Hong Ta-gu, a Koryŭ renegade, took advantage of this to charge the Koryŭ prince, who was then in Peking, with having insulted the Mongol crown prince. The emperor believed the charge and cut off the Koryŭ prince’s revenues and treated him with marked coldness. Hong also poisoned the emperor toward Koryŭ by intimating that she would soon attempt to throw off the yoke of China. But by the following year the relations seem to have become cordial again, for when the king asked that the tribute be remitted on the ground of the heavy expense of rebuilding palaces at Song-do, the emperor not only consented but sent a present of 500 sheep. Koryŭ was also fortunate in the sending of an envoy to Japan, for he returned with a large amount of rice and cloth from Tsushima, which had been stolen by Japanese corsairs.
In 1263 the king was ordered to repair to Peking. A long discussion followed, some of the courtiers advising one thing and some another. The monks at this time said, in effect, “I told you so”, for they had long ago promised the king that if he would favor them he would not be called to Peking. But go he did, leaving his son to administer the kingdom in his absence. Sun, whom we will remember as the Koryŭ gentleman who had married a Mongol princess and who was thoroughly Mongolized, told the emperor that there were 38,000 troops in Koryŭ and that someone should go and bring them to China where they could act as allies for the Mongols in their conquests. To this Yi Chang-yung, who was in the king’s retinue, answered. “Formerly we had that number of soldiers but many have died and few are left. If the emperor cannot believe this let him send Sun with me to Koryŭ and we will review all the troops and learn the truth.” This was a telling blow, for Sun knew that if he once crossed into Koryŭ territory his life would not be worth an hour’s ransom; so he discreetly held his peace. The king came back to Song-do in December of the same year.