Būkū Khān, having learnt in a dream that he would possess the entire world, assembled his troops and sent his brothers to wage war against the Mongols, Kirghiz, Tanguts, and Khitāys.[247] They returned to their dwelling-place with great booty, and founded the city of Urdu Bālik. Būkū Khan again dreamt that a piece of jade was given him with the assurance that as long as he preserved it he would rule the world. The prospect induced him to turn his arms to the west and enter Turkestān, where he built the city of Balāsāghūn.[248] We know from Chinese sources that these Uïghūrs[249] had their abode in the seventh century in the north-west of Mongolia; that in the eighth century they dwelt near the place where, in the five hundred years later, the Mongols built Karakorum. In the ninth century their empire in Mongolia was destroyed by the Kirghiz, when they were dispersed, and apparently split into two parties. The eastern branch came into contact with Chingiz Khān. After and thenceforward they appear in the Mongol-Chinese annals as under the name of Wei-wu-rh.[250] Of the Western Uïghūrs little is known, but they may be identified with the Eastern Turks of Mohammedan authors of the eleventh and twelfth centuries.[251] The first of the Uïghūr Khāns of Turkestān who plays any great part in Mohammedan history is Boghrā Khān, whose capital was Balāsāghūn, and who ruled over Kāshghar (called Urdu Kend), Khotan, Karakorum, Tarās, and Fārāb (Otrār).[252] He was contemporaneous with Nūh III., seventh of the Sāmānide line, whose reign was characterised by the utmost confusion and anarchy. Two of Nūh’s most powerful nobles--Abū `Alī Sīmjūr, governor of Khorāsān, and Fā´ik, governor of Herāt, whose insubordination had received severe but well-merited punishment at the hands of their master--made treacherous overtures to Boghrā Khān, and invited him to attack Nūh. The invitation was accepted with alacrity by the Uïghūr prince, who at once set out for Samarkand, which was delivered over to him by the faithless Fā´ik, whom Nūh had intrusted with its defence. The feeble representative of the Sāmānides, thus betrayed, fled from his capital, which Boghrā Khān shortly after entered in triumph, and became practically master of Transoxiana. But the climate of Bokhārā did not suit him. He set out for his home, when death overtook him ere he had gone many stages, A.H. 383 (993). Meanwhile Nūh re-entered Bokhārā and regained possession of his dominion. But though he was heartily welcomed by the people he did not feel secure from the treachery of his nobles, and on this account he invited the great Sabuktagin, in the year A.H. 384 (994), to come to his aid. Sabuktagin immediately hastened over the mountain passes at the head of 20,000 men, and, crossing the Oxus, joined Nūh at Kesh. Seconded by his son Mahmūd, he gained three victories over the rebel lieutenants at Herāt, Nīshāpūr, and finally at Tūs. Fā´ik had in the