Iran, also called Persia, and officially the Islamic Republic of Iran, is a country in Western Asia.With 82 million inhabitants, Iran is the world's 18th most populous country. Its territory spans 1,648,195 km2 (636,372 sq mi), making it the second largest country in the Middle East and the 17th largest in the world. Iran is bordered to the northwest by Armenia and the Republic of 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. Its central location in Eurasia and Western Asia, and its proximity to the Strait of Hormuz, give it geostrategic importance. Tehran is the political and economic center of Iran, and the largest and most populous city in Western Asia with more than 8.8 million residents in the city and 15 million in the larger metropolitan area.
Iran is home to one of the world's oldest civilizations, beginning with the formation of the Elamite kingdoms in the fourth millennium BCE. It was first unified by the Iranian Medes in the seventh century BCE, and reached its territorial height in the sixth century BCE under Cyrus the Great, whose Achaemenid Empire stretched from Eastern Europe to the Indus Valley, one of the largest empires in history. The empire fell to Alexander the Great in the fourth century BCE and was divided into several Hellenistic states. An Iranian rebellion established the Parthian Empire in the third century BCE, which was succeeded in the third century CE by the Sasanian Empire, a leading world power for the next four centuries