Kyrgyzstan's history spans a variety of cultures and empires. Although geographically isolated by its highly mountainous terrain, Kyrgyzstan has been at the crossroads of several great civilizations as part of the Silk Road and other commercial routes. Inhabited by a succession of tribes and clans, Kyrgyzstan has periodically fallen under larger domination. Between periods of self-government it was ruled by Göktürks, the Uyghur Empire and the Khitan people, before being conquered by the Mongols in the 13th century; it regained independence but was invaded by Kalmyks, Manchus and Uzbeks. In 1876, it became part of the Russian Empire, remaining in the USSR as the Kirghiz Soviet Socialist Republic after the Russian Revolution. Following Mikhail Gorbachev's democratic reforms in the USSR, in 1990 pro-independence candidate Askar Akayev was elected president. On 31 August 1991, Kyrgyzstan declared independence from Moscow and a democratic government was established. Kyrgyzstan attained sovereignty as a nation state after the breakup of the Soviet Union in 1991.