Conquests in Northern China

Genghis Khan moved to secure his borders. To his south he made an alliance with the Uighurs, who were closer than the Mongols were to the Silk Road and to wealth. He married his daughter to the Uighur Khan, and the Uighur Khan brought to the wedding party a caravan laden with gold, silver, pearls, brocaded fabrics, silks and satins. The Mongols had only leather, fur and felt -- a humiliation for a master of the entire world.
Genghis Khan needed booty to pay troops securing his northern border and subduing an old enemy there, the Merkits. He acted on his mandate as the rightful ruler of the entire world and attacked the rulers of farmers and herders in northwestern China, the Tangut, who had much in goods like the Uighur Khan. In warriors the Mongols were outnumbered two to one, and they had to learn a new kind of warfare against fortified cities, including cutting supply lines and diverting rivers. Genghis Khan and his army were victorious, and in 1210 Genghis Khan won from the Tangut recognition as overlord.
Also in 1210, the Jin dynastry of Jurchen people, who ruled that part of northern China that included Beijing, sent a delegation to Genghis Khan demanding Mongol submission as vassals. The Jin dynasty controlled the flow of goods along the Silk Road, and defying them meant a lack of access to those goods. Genghis Khan and the Mongols discussed the matter and chose war. Genghis, according to the scholar Jack Weatherford, prayed alone on a mountain, bowing down and stating his case to "his supernatural guardians," describing the grievances, the tortures and killings that generations of his people had suffered at the hands of the Jurchen. And he pleaded that he had not sought war against the Jurchen and had not initiated the quarrel. [note]

Genghis Khan and the Making of the Modern World, by Jack Weatherford, p. 83.

In 1211, Genghis Khan and his army attacked. The Jurchen (Jin) dynasty had a large and effective army but they were hard pressed by both the Mongols and by a border war with the Tangut. They were also under attack by Chinese from south of the Yangzi River, the Southern Song emperor wishing to take advantage of the Jurchen-Mongol conflict to liberate northern China. But the Jurchen drove the Chinese armies into retreat. The Mongols were benefiting from China having failed during the previous century to make itself a strong military power. They benefited too from the Jurchen (Jin) dynastry ruling conquered people. The Mongols used divide and conquer tactics, using benevolence toward those who sided with them and terror and bloodshed against those who did not. They ravaged the countryside, gathering information and booty and driving populations in front of them, clogging the roads and trapping the Jurchen within their cities, where the Jurchen (Jin) dynasty was subject to revolts. The Mongols used conscripted labor in attacking cities and in operating their newly acquired Chinese siege engines.
Against the Jurchen the Mongols had an advantage in diet, which included a lot of meat, milk and yogurt, and they could miss a day or two of eating better than Jurchen soldiers, who ate grains. Genghis Khan and his army overran Beijing and pushed into the heartland of northern China. Military success helped as people acquired the impression that Genghis Khan had the Mandate of Heaven and that fighting against him was fighting heaven itself. The Jurchen emperor recognized Mongol authority and agreed to pay tribute.
After six years of fighting the Jurchen, Genghis Khan returned to Mongolia, leaving one of his best generals in charge of Mongol positions. Returning with Genghis Khan and his Mongols were engineers who had become a permanent part of their army, and there were captive musicians, translators, doctors and scribes, camels and wagonloads of goods. Among the goods were silk, including silken rope, cushions, blankets, robes, rugs, wall hangings, porcelain, iron kettles, armor, perfumes, jewelry, wine, honey, medicines, bronze, silver and gold, and much else. And goods from China would now come in a steady flow.
The Mongols were happy to be back from China, their homeland higher in elevation, less humid and cooler. As eaters of meat and sparsely populated they felt superior to people in northern China, but they liked what China had to offer, and at home there was change. The continuing flow of goods from China had to be administered and properly distributed, and buildings had to be built to store the goods. Success in war was changing the Mongols - as it had the Romans and the Arabs.