The 18th-largest country in the world in terms of area at 1,648,195 km2 (636,372 sq mi), Iran is an ethnically diverse country and has a population of around 77 million. It is a mountainous country of particular geopolitical significance owing to its location in three spheres of Asia. Tehran is the capital, the country's most populous city and the political, cultural, commercial and industrial center of the nation. Iran is a regional power, and holds an important position in international energy security and world economy as a result of its large reserves of petroleum and natural gas. Iran has the largest proven natural gas reserves in the world, and the fourth largest proven petroleum reserves.
Iran is home to one of the world's oldest civilizations. The first dynasty in Iran formed during the Elamite kingdom in 2800 BC. The Iranian Medes unified Iran into an empire in 625 BC. Cyrus the Great founded the first Achaemenid Empire (550–330 BC), which became the largest ancient empire. In 633 AD, Muslim armies invaded Iran, and had conquered the region by 651 AD. The emergence in 1501 of the Safavid dynasty, which promoted Twelver Shia Islam as the official religion of their empire, marked one of the most important turning points in Iranian and Muslim history. The Persian Constitutional Revolution established the nation's first parliament in 1906, within a constitutional monarchy. Following a coup d'état instigated by the UK and US in 1953, Iran gradually became a more autocratic country. Growing dissent with foreign influence culminated during the Iranian Revolution which led to the establishment of an Islamic republic on 1 April 1979.
Iran is a founding member of the UN, NAM, OIC and OPEC. The political system of Iran, based on the 1979 constitution, combines elements of a parliamentary democracy with a religious theocracy run by the country’s clergy. The highest state authority is the Supreme Leader. Shia Islam is the official religion and Persian is the official language.