The capture of Yenking, where Genghis obtained a large supply of war materials, as well as vast booty, opened the road to Central China. The Mongols advanced as far as the celebrated Tunkwan Pass, which connects Shensi and Honan, but when their general, Samuka, saw how formidable it was, and how strong were the Kin defenses and garrison, he declined to attack it, and, making a detour through very difficult country, he marched on Kaifong, where Utubu little expected him. The Mongols had to make their own road, and they crossed several ravines by improvised "bridges made of spears and the branches of trees bound together by strong chains." But the Mongol force was too small to accomplish any great result, and the impetuosity of Samuka nearly led to his destruction. A prompt retreat, and the fact that the Hoangho was frozen over, enabled him to extricate his army, after much fatigue and reduced in numbers, from its awkward position. The retreat of the Mongols inspired Utubu with sufficient confidence to induce him to attack Yeliu Liuko in Leaoutung, and the success of this enterprise imparted a gleam of sunshine and credit to the expiring cause of the Kins. Yeliu Liuko was driven from his newly-created kingdom, but Genghis hastened to the assistance of his ally by sending Muhula, the greatest of all his generals, at the head of a large army to recover Leaoutung. His success was rapid and remarkable. The Kins were speedily overthrown, Yeliu Liuko was restored to his authority, and the neighboring King of Corea, impressed by the magnitude of the Mongol success, hastened to acknowledge himself the vassal of Genghis. The most important result of this campaign was that Genghis intrusted to Muhula the control of all military arrangements for the conquest of China. He is reported to have said to his lieutenant: "North of the Taihing Mountains I am supreme, but all the regions to the south I commend to the care of Muhula," and he "also presented him with a chariot and a banner with nine scalops. As he handed him this last emblem of authority, he spoke to his generals, saying, 'Let this banner be an emblem of sovereignty, and let the orders issued from under it be obeyed as my own.'" The principal reason for intrusting the conquest of China to a special force and commander was that Genghis wished to devote the whole of his personal attention to the prosecution of his new war with the King of Khwaresm and the other great rulers of Western Asia.