Claim: The world's 18th-largest country is Iran.

Iran -LRB- ɪ , also ɪ -RRB- , formerly known as Persia , is a sovereign state in Western Asia . It is bordered to the northwest by Armenia , the de facto Republic of Artsakh and Azerbaijan ; to the north by the Caspian Sea ; to the northeast by Turkmenistan ; to the east by Afghanistan and Pakistan ; to the south by the Persian Gulf and the Gulf of Oman ; and to the west by Turkey and Iraq . Comprising a land area of 1648195 km2 , it is the second-largest country in the Middle East and the 18th-largest in the world . Officially with 79.93 million inhabitants -LRB- Nov 2016 -RRB- , Iran is the world 's 18th-most-populous country . It is the only country with both a Caspian Sea and an Indian Ocean coastline . The country 's central location in Eurasia and Western Asia , and its proximity to the Strait of Hormuz , make it of great geostrategic importance . Iran has diverse landscapes and geographical conditions . Tehran is the country 's capital and largest city , as well as its leading economic and cultural center .   Iran is home to one of the world 's oldest civilizations , beginning with the formation of the Elamite kingdoms in the 4th millennium BC . The area was first unified and dominated by the Iranian Medes in the 7th century BC . Iran reached its greatest extent during the Achaemenid Empire -LRB- First Persian Empire -RRB- founded by Cyrus the Great in 6th century BC , the largest empire the world had yet seen , stretching from Eastern Europe to the Indus Valley . Persia fell to Alexander the Great in 4th century BC , but reemerged 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 faiths of Zoroastrianism and Manichaeism with the new religion of 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 cultural entity . 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 . Conflicts 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 increasingly autocratic . Growing dissent against foreign influence and political repression led to the 1979 Revolution and the establishment of an Islamic republic , a political system which combines elements of a parliamentary democracy with a theocracy governed by Islamic jurists under the concept of a `` Supreme Leadership '' . According to international observers , the current Iranian government is oppressive , with human rights abuses commonplace .   A multicultural country comprising numerous ethnic and linguistic groups , most inhabitants of Iran are Shia Muslims and Persian is the official language . The largest ethnic groups in Iran are the Persians , Azeris , Kurds and Lurs .   Iran is a founding member of the UN , ECO , NAM , OIC , and OPEC . Iran is a major regional and middle power , and its large reserves of fossil fuels -- which include the largest natural gas supply in the world and the fourth-largest proven oil reserves -- exert considerable influence in international energy security and the world economy . Iran 's rich cultural legacy is reflected in part by its 21 UNESCO World Heritage Sites , the third-largest number in Asia and 11th-largest in the world .