often declined as quickly as they formed. Once the foreign powers were expelled, several indigenous empires formed in Central Asia. The Hephthalites were the most powerful of these nomad groups in the 6th and 7th century and controlled much of the region. In the 10th and 11th centuries the region was divided between several powerful states including the Samanid dynasty, that of the Seljuk Turks, and the Khwarezmid Empire. The most spectacular power to rise out of Central Asia developed when Genghis Khan united the tribes of Mongolia. Using superior military techniques, the Mongol Empire spread to comprise all of Central Asia and China as well as large parts of Russia, and the Middle East. After Genghis Khan died in 1227, most of Central Asia continued to be dominated by the Mongol successor Chagatai Khanate. This state proved to be short lived, as in 1369 Timur, a Turco-Mongol ruler, conquered most of the region. Even harder than keeping a steppe empire together was governing conquered lands outside the region. While the steppe peoples of Central Asia found conquest of these areas easy, they found governing almost impossible. The diffuse political structure of the steppe confederacies was maladapted to the complex states of the settled peoples. Moreover, the armies of the nomads were based upon large numbers of horses, generally three or four for each warrior. Maintaining these forces required large stretches of grazing land, not present outside the steppe. Any extended time away from the homeland would thus cause the steppe armies to gradually disintegrate. To govern settled peoples the steppe peoples were forced to rely on the local bureaucracy, a factor that would lead to the rapid assimilation of the nomads into the culture of those they had conquered. Another important limit was that the armies, for the most part, were unable to penetrate the forested regions to the north; thus, such states as Novgorod and Muscovy began to grow in power. In the 14th century much of Central Asia, and many areas beyond it, were conquered by Timur (1336–1405) who is known in the west as Tamerlane. It was during Timur's reign that the nomadic steppe culture of Central Asia fused with the settled culture of Iran. One of its consequences was an entirely new visual language that glorified Timur and subsequent Timurid rulers. This visual language was also used to articulate their commitment to Islam. Timur's large