Iran is home to one of the world's oldest civilizations, beginning with the formation of the Elamite kingdoms in the fourth millennium BC. It was first unified by the Iranian Medes in the seventh century BC, and reached its territorial height in the sixth century BC, when Cyrus the Great founded the Achaemenid Empire, which became one of the largest empires in history and has been described as the world's first superpower. The empire fell to Alexander the Great in the fourth century BC and was divided into several Hellenistic states. An Iranian rebellion established the Parthian Empire in the third century BC, which was succeeded in the third century AD by the Sasanian Empire, a major world power for the next four centuries. Arab Muslims conquered the empire in the seventh century AD, which led to the Islamization of Iran. It subsequently became a major center of Islamic culture and learning, with its art, literature, philosophy, and architecture spreading across the Muslim world and beyond during the Islamic Golden Age. Over the next two centuries, a series of native Muslim dynasties emerged before the Seljuq Turks and the Mongols conquered the region. In the 15th century, the native Safavids re-established a unified Iranian state and national identity and converted the country to Shia Islam. Under the reign of Nader Shah in the 18th century, Iran once again became a major world power, though by the 19th century a series of conflicts with Russia led to significant territorial losses. The early 20th century saw the Persian Constitutional Revolution. Efforts to nationalize its fossil fuel supply from Western companies led to an Anglo-American coup in 1953, which resulted in greater autocratic rule under Mohammad Reza Pahlavi and growing Western political influence. He went on to launch a far-reaching series of reforms in 1963. After the Iranian Revolution, the current Islamic Republic was established in 1979 by Ruhollah Khomeini, who became the country's first Supreme Leader.