Uzbekistan is a landlocked country in Central Asia. The most populous state in the region, it is the only country to border all other Central Asian states. Uzbekistan is a significant producer and exporter of oil and natural gas which, along with agriculture, dominate the country's economy. These resources, as well as the country's strategic location and the presence of militant groups, have brought the interest of foreign powers such as China, Russia and the United States. Because of this, Uzbekistan's primary geographic challenge is to maintain unity while also balancing against its regional neighbors and external powers. Historically, Uzbekistan was an important component of the Silk Road, and cities such as Samarkand, Bukhara and Khiva are still key political and population centers in the country. After Uzbekistan became overrun by the Mongols, it became the seat of an expansive Timurid dynasty. The territory of Uzbekistan was then divided between a series of Khanates before being absorbed into the Russian Empire in the 19th century. A former member of the Soviet Union, the landlocked country gained independence in 1991. Modern day Uzbekistan is centered around its capital in Tashkent, though the country has substantial regional divisions. While the western half is largely sparsely populated desert, the majority of the population is concentrated in the demographic and agricultural heartland of Central Asia, the Fergana Valley. The valley itself is divided by a series of complex and poorly defined borders established during the Soviet era to prevent the emergence of a unified Central Asian state. These borders have led to ethnic tensions, resource disputes, and the frequent border skirmishes between Uzbekistan and neighboring states such as Kyrgyzstan and Tajikistan.