Iran was the cradle of one of the most ancient civilizations in the world, starting with the formation of the Elamite Empire in the 4th millennium BC. The area was first ruled by the Iranian Medes in the 7th century BC. Iran reached its greatest size during the Achaemenid Empire founded by Cyrus the Great in 6th century BC, the largest empire the world had yet seen, stretching from Eastern Europe to India. Persia fell to Alexander the Great in 4th century BC, but rose again shortly after as the Parthian Empire. Under the Sassanians, Persia became a leading world power for the next four centuries.
In 7th century AD, Arabs conquered Persia and largely displaced the indigenous faith of Zoroastrianism with Islam. Iranians became major contributors to the Islamic Golden Age that followed, producing many influential scientists, scholars, artists and thinkers. After two centuries, a period of various Iranian dynasties began, although these dynasties were later conquered by Turks and then the Mongols. An Iranian national identity was always reasserted however, developing a distinct culture. The rise of the Safavids in 15th century led to the establishment of Twelver Shia Islam as the official religion, marking a turning point in Muslim history. In the 18th century, Iran arguably possessed briefly the most powerful military at the time under Nader Shah. The wars with the Russian Empire in 19th century led to significant territorial losses. Popular unrest culminated in the Persian Constitutional Revolution, which established a constitutional monarchy and the country's first legislative body, the Majles. Following a coup d'état instigated by the UK and the US in 1953, Iran gradually became closely aligned with the West and grew very autocratic. Growing dissent against foreign influence and political repression led to the 1979 Islamic Revolution and the establishment of The Islamic Republic of Iran, a political system which includes elements of a parliamentary democracy vetted and supervised by the country’s Supreme Leader.