Iran (/aɪˈræn/ or /ɪˈrɑːn/; Persian: ایران‎‎ – Irān [ʔiːˈɾɒːn] ), also known as Persia (/ˈpɜːrʒə/ or /ˈpɜːrʃə/), officially the Islamic Republic of Iran (جمهوری اسلامی ایران – Jomhuri ye Eslāmi ye Irān [d͡ʒomhuːˌɾiːje eslɒːˌmiːje ʔiːˈɾɒːn]), is a sovereign state in Western Asia. It is bordered to the northwest by Armenia, the de facto Nagorno-Karabakh Republic, and Azerbaijan; to the northeast by Turkmenistan; to the east by Afghanistan and Pakistan; to the south by the Persian Gulf and the Gulf of Oman; to the north by the Caspian Sea; and to the west by Turkey and Iraq. Comprising a land area of 1,648,195 km2 (636,372 sq mi), it is the second-largest country in the Middle East and the 18th-largest in the world. With 78.4 million inhabitants, Iran is the world's 17th-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. Tehran is the country's capital and largest city as well as its leading economic center. Iran is heir to one of the world's oldest civilizations, beginning with the formation of the Proto-Elamite and Elamite kingdoms in 3200–2800 BC. The area was first unified by the Iranian Medes in 625 BC, who became the dominant cultural and political power in the region. Iran reached its greatest geographic extent during the Achaemenid Empire founded by Cyrus the Great in 550 BC, which at one time stretched from parts of Eastern Europe in the west, to the Indus Valley in the east, making it the largest empire the world had yet seen. The empire collapsed in 330 BC following the conquests of Alexander the Great, but reemerged shortly after as the Parthian Empire. Under the Sassanid Dynasty, Iran again became one of the leading powers in the world for the next four centuries. Beginning in 633 AD, Rashidun Arabs conquered Iran and largely displaced the indigenous faiths of Manichaeism and Zoroastrianism by Sunni Islam. Iran became a major contributor to the Islamic Golden Age that followed, producing many influential scientists, scholars, artists, and thinkers. The rise of the Safavid Dynasty in 1501 led to the establishment of Twelver Shia Islam as the official religion of Iran, marking one of the most important turning points in Iranian and Muslim history. During the 18th century, Iran reached its greatest territorial extent since the Sassanid Empire, and under Nader Shah briefly possessed what was arguably the most powerful empire at the time. Through the late 18th and 19th centuries, a series of conflicts with Russia led to significant territorial losses and the erosion of sovereignty. Popular unrest culminated in the Persian Constitutional Revolution of 1906, which established a constitutional monarchy and the country's first legislative body, the Majles. Following a coup d'état instigated by the U.K. and the U.S. in 1953, Iran gradually became closely aligned with the United States and the rest of the West but grew increasingly autocratic. Growing dissent against foreign influence and political repression led to the 1979 Revolution and the establishment of an Islamic republic. 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. Iran is a founding member of the UN, ECO, NAM, OIC, and OPEC. Its political system is based on the 1979 Constitution which combines elements of a parliamentary democracy with a theocracy governed by Islamic jurists under the concept of a Supreme Leadership. A multicultural country comprising numerous ethnic and linguistic groups, most inhabitants are Shia Muslims and Persian is the official language.