In the thirteenth century, Genghis became Khan[15] of all the Mongols, who under him were once more the terror of the world. Genghis had already conquered the Naimans, a powerful people in their day; and invaded Tangout. When he had assembled under his standard the tribes of his own people and of the nations whom he had conquered, he was impelled by his restless ambition to keep them in motion. The lust of conquest became his ruling passion, and every new trophy added fuel to its flame. He first invaded Kitai, or northern China, overran the territories of the then powerful Kin, desolated their cities and villages, and massacred their people, and then retired to the river Tolla to recruit, having added to his army many Chinese of all ranks. The seven years' campaign in the west followed shortly after, during which Genghis conquered Persia and Bukhara, destroyed many populous cities, and put to the sword prodigious numbers of human beings. His lieutenants extended their ravages still further westward, while Genghis himself returned to his head-quarters at Kara-Korum. Kitai was again invaded, and Tangout subjugated. On the death of Genghis, in 1227, the succession to the Khanate fell to his son Oktai, who followed up the conquest of China according to instructions delivered by Genghis on his death-bed. But the empire had become so unwieldy, and the distances that separated the divisions of it so vast, that it could no longer subsist in its integrity. It was soon split up into sections, which were parcelled out to the descendants of Genghis. Some reigned in Persia, and some in Kapchak, a territory stretching from the Caspian Sea to Kazan, and covering a large portion of the steppe of the Kirghis. The little dynasty of the Nogai Tartars was also founded in Europe by a descendant of Genghis. The Tartar kingdoms of Kazan and the Crimea were both offshoots from the khans of Kapchak. Batou, Khan of Kapchak, or the Golden Horde, took Moscow and wasted the Russian provinces. Kublai, who succeeded to China, was the greatest of them all. In addition to that country, he possessed Pegu, Thibet, and the whole of Tartary; while Cochin China, Tonquin, and Corea paid him tribute. He was, moreover, acknowledged by all the other khans as their chief. But the whole continent of Asia lay between him and his vassals, and his suzerainty soon became a name only, and in course of time the form also was discontinued. [15] The title of Khan was first assumed by the Geougen, in the fifth century. The Mongols were, however, incapable of maintaining a settled government. The expeditions to subjugate Japan having proved fruitless, there was no other country left for them to conquer; this quiescent state was unnatural to them, and Chinese culture demoralised them in less than a hundred years.