In 1206, Yezokai—the word in Japanese means Yezo Sea—the leader of the Mongols, at the request of his chieftains, took the name of Genghis Khan and proclaimed himself the ruler of an empire. He now set before himself the task of subduing the Kitans and absorbing their land and people, preparatory to the conquest of China. This was accomplished in less than six years. Liao Tung was invaded and, in 1213, his armies were inside the Great Wall. Three mighty hosts were now organized, one to overrun all China to Nepal and Anam, one to conquer Corea and Japan, and one to bear the white banners of the Mongols across Asia into Europe. This work, though not done in a day, was nearly completed before a generation passed. [5] Genghis Khan led the host that moved to the west. In 1218 the Corean king declared himself a vassal of Genghis. In 1231 the murder of a Mongol envoy in Corea was the cause of the first act of war. The Mongols invaded the country, captured forty of the principal towns, received the humiliation of the king, who had fled to Kang-wa Island, and began the abolition of Corean independence by appointing seventy-two Mongol prefects to administer the details of local government. The people, exasperated by the new and strange methods of their foreign conquerors, rose against them and murdered them all. This was the signal for a second and more terrible invasion. A great Mongol army overran the country in 1241, fought a number of pitched battles, defeated the king, and again imposed heavy tribute on their humbled vassal. In 1256 the Corean king went in person to do homage at the court of the conqueror of continents. In the details of the Mongol rule kindness and cruelty were blended. The most relentless military measures were taken to secure obedience after the conciliatory policy failed. By using both methods the great Khan kept his hold on the little peninsula, although the Coreans manifested a constant disposition to revolt.