In 1207 Genghis began his war with the state of Hia, which he had determined to crush as the preliminary to an invasion of China. In that year he contented himself with the capture of Wuhlahai, one of the border fortresses of that principality, and in the following year he established his control over the tribes of the desert more fully, thus gaining many Kirghiz and Naiman auxiliaries. In 1209 he resumed the war with Hia in a determined spirit, and placed himself in person at the head of all his forces. Although the Hia ruler prepared as well as he could for the struggle, he was really unnerved by the magnitude of the danger he had to face. His army was overthrown, his best generals were taken prisoners, and he himself had no resource left but to throw himself on the consideration of Genghis. For good reasons the Mongol conqueror was lenient. He married one of the daughters of the king, and he took him into subsidiary alliance with himself. Thus did Genghis absorb the Hia power, which was very considerable, and prepared to enroll it with all his own resources against the Kin empire. If the causes of Mongol success on this occasion and afterward are inquired for, I cannot do better than repeat what I previously wrote on this subject: "The Mongols owed their military success to their admirable discipline and to their close study of the art of war. Their military supremacy arose from their superiority in all essentials as a fighting power to their neighbors. Much of their knowledge was borrowed from China, where the art of disciplining a large army and maneuvering it in the field had been brought to a high state of perfection many centuries before the time of Genghis. But the Mongols carried the teaching of the past to a further point than any of the former or contemporary Chinese commanders, indeed, than any in the whole world, had done; and the revolution which they effected in tactics was not less remarkable in itself, and did not leave a smaller impression upon the age, than the improvements made in military science by Frederick the Great and Napoleon in their day. The Mongol played in a large way in Asia the part which the Normans on a smaller scale played in Europe. Although the landmarks of their triumph have now almost wholly vanished, they were for two centuries the dominant caste in most of the states of Asia."
Having thus prepared the way for the larger enterprise, it only remained to find a plausible pretext for attacking the Kins. With or without a pretext Genghis would no doubt have made war, but even the ruthless Mongol sometimes showed a regard for appearances. Many years before the Kins had sent as envoy to the Mongul encampment Chonghei, a member of their ruling house, and his mission had been not only unsuccessful, but had led to a personal antipathy between the two men. In the course of time Chonghei succeeded Madacou as emperor of the Kins, and when a Kin messenger brought intelligence of this event to Genghis, the Mongol ruler turned toward the south, spat upon the ground, and said, "I thought that your sovereigns were of the race of the gods, but do you suppose that I am going to do homage to such an imbecile as that?" The affront rankled in the mind of Chonghei, and while Genghis was engaged with Hia, he sent troops to attack the Mongol outposts. Chonghei thus placed himself in the wrong, and gave Genghis justification for declaring that the Kins and not he began the war. The reputation of the Golden dynasty, although not as great as it once was, still stood sufficiently high to make the most adventurous of desert chiefs wary in attacking it. Genghis had already secured the co-operation of the ruler of Hia in his enterprise, and he next concluded an alliance with Yeliu Liuko, chief of the Khitans, who were again manifesting discontent with the Kins. Genghis finally circulated a proclamation among all the desert tribes, calling upon them to join him in his attack on the common enemy. This appeal was heartily and generally responded to, and it was at the head of an enormous force that Genghis set out in March, 1211, to effect the conquest of China. The Mongol army was led by Genghis in person, and under him his four sons and his most famous general, Chepe Noyan, held commands.
The plan of campaign of the Mongol ruler was as simple as it was bold. From his camp at Karakoram, on the Kerulon, he marched in a straight line through Kuku Khoten and the Ongut country to Taitong, securing an unopposed passage through the Great Wall by the defection of the Ongut tribe. The Kins were unprepared for this sudden and vigorous assault directed on their weakest spot, and successfully executed before their army could reach the scene. During the two years that the forces of Genghis kept the field on this occasion, they devastated the greater portion of the three northern provinces of Shensi, Shansi, and Pechihli. But the border fortress of Taitong and the Kin capital, Tungking, successfully resisted all the assaults of the Mongols, and when Genghis received a serious wound at the former place, he reluctantly ordered the retreat of his army, laden with an immense quantity of spoil, but still little advanced in its main task of conquering China. The success of the Khitan Yeliu Liuko had not been less considerable, and he was proclaimed King of Leaou as a vassal of the Mongols. The planting of this ally on the very threshold of Chinese power facilitated the subsequent enterprises of the Mongols against the Kins, and represented the most important result of this war.