Iran is home to one of the world's oldest civilizations, beginning with the formation of the Elamite kingdoms in the fourth millennium BC. It was first unified by the Iranian Medes in the seventh century BC, and reached its greatest extent during the Achaemenid Empire founded by Cyrus the Great in the sixth century BC, stretching from Eastern Europe to the Indus Valley, becoming a larger empire than previously ever existed in the world. The Iranian realm fell to Alexander the Great in the fourth century BC, but reemerged shortly after as the Parthian Empire, followed by the Sasanian Empire, which became a leading world power for the next four centuries.
Arab Muslims conquered the empire in the seventh century AD, ultimately leading to the displacement of the indigenous faiths of Zoroastrianism and Manichaeism with Islam. Iran made major contributions to the Islamic Golden Age that followed, producing many influential figures in art and science. After two centuries, a period of various native Muslim dynasties began, which were later conquered by the Turks and the Mongols. The rise of the Safavids in the 15th century led to the reestablishment of a unified Iranian state and national identity, which followed the country's conversion to Shia Islam, marking a turning point in Iranian and Muslim history. By the 18th century, under Nader Shah, Iran briefly possessed what was arguably the most powerful empire at the time. The 19th-century conflicts with the Russian Empire led to significant territorial losses. Popular unrest culminated in the Constitutional Revolution of 1906, which established a constitutional monarchy and the country's first legislature. Following the coup of 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, which followed the establishment of an Islamic republic, a political system which includes elements of a parliamentary democracy vetted and supervised by a theocracy governed by an autocratic "Supreme Leader". During the 1980s, the country was engaged in a war with Iraq, which lasted for almost nine years and resulted in a high number of casualties and financial loss for both sides.
According to international observers, Iran's human rights record has rapidly deteriorated, with severe human rights violations being committed by the government against Iranian dissidents; in several cases, several opponents of the current Supreme Leader Ali Khamenei have been tortured, imprisoned, sentenced to death or executed for insulting him. Iran has responded to denunciation of its human rights record by verbally attacking its critics and harshly criticizing them, leading to tensions and anger toward Iran from the West. Since the 2000s, Iran's controversial nuclear program has raised concerns, which is part of the basis of the international sanctions against the country. The Joint Comprehensive Plan of Action, an agreement reached between Iran and the P5+1, was created on July 14, 2015, aimed to loosen the nuclear sanctions in exchange for Iran's restriction in producing enriched uranium.
Iran is a founding member of the UN, ECO, NAM, OIC, and OPEC. It is a major regional and middle power, and its large reserves of fossil fuels – which include the world's largest natural gas supply and the fourth-largest proven oil reserves – exert considerable influence in international energy security and the world economy.
The country's rich cultural legacy is reflected in part by its 22 UNESCO World Heritage Sites, the third-largest number in Asia and eleventh-largest in the world. Iran is a multicultural country comprising numerous ethnic and linguistic groups, the largest being Persians (61%), Azeris (16%), Kurds (10%), and Lurs (6%).