Iran,[a] also known as Persia[b] and officially the Islamic Republic of Iran (IRI),[c] is a country in West Asia. It is bordered by Iraq to the west and Turkey to the northwest, Azerbaijan, Armenia, the Caspian Sea and Turkmenistan to the north, Afghanistan to the east, Pakistan to the southeast, the Gulf of Oman and the Persian Gulf to the south. With a mostly Persian-ethnic population of almost 90 million in an area of 1,648,195 km2 (636,372 sq mi), Iran ranks 17th globally in both geographic size and population. It is the sixth-largest country entirely in Asia, the second-largest in West Asia, and one of the world's most mountainous countries. Officially an Islamic republic, Iran has a Muslim-majority population. The country is divided into five regions with 31 provinces. The nation's capital and most populous city is Tehran, with around 16 million people in its metropolitan area. Other major cities include Mashhad, Isfahan, Karaj, and Shiraz.
Iran is one of the world's oldest civilizations, beginning with the Elamites in the fourth millennium BC. It was first unified as a state by the Medes in the seventh century BC and reached its territorial height in the sixth century BC, when Cyrus the Great founded the Achaemenid Empire, one of the largest and most powerful empires in ancient history. Alexander the Great conquered the empire in the fourth century BC, subsequently dividing Iran into several Hellenistic states. An Iranian rebellion established the Parthian Empire in the third century BC, which was succeeded in the third century AD by the Sasanian Empire. Muslims conquered the region in the seventh century AD, leading to Iran's Islamization. Iran became a major centre of Islamic culture and learning, and its culture, language, and customs spread across the Muslim world. A series of native Iranian Muslim dynasties ruled the country until the Seljuk and the Mongol conquests of the 11th to 14th centuries. In the 16th century, the native Safavids re-established a unified Iranian state with Twelver Shia Islam as the official religion, marking the beginning of modern Iranian history.
During the Afsharid Empire in the 18th century, Iran was a leading world power, though by the 19th century, it had lost significant territory through a series of conflicts with the Russian Empire. The early 20th century saw the Persian Constitutional Revolution, the establishment of the Pahlavi dynasty, and efforts at modernization. Attempts by Mohammad Mosaddegh to nationalize the country's vast fossil fuel supply led to an Anglo-American coup in 1953. After the Iranian Revolution, the monarchy was overthrown in 1979 and the Islamic Republic of Iran was established by The Grand Ayatollah Ruhollah Khomeini, who became the country's first supreme leader. The forces of Saddam Hussein invaded the Iranian province of Khuzestan in 1980, initiating the Iran-Iraq War, which by the end of it in 1988, became the longest conventional war of the 20th century. Iran is officially governed as an Islamic Republic with a presidential system, albeit with ultimate authority vested in a supreme leader (rahbar), currently Ali Khamenei since Khomeini's death in 1989. The Iranian government has attracted criticism for its constraints and violations of human rights.