Once a major empire of superpower proportions, having conquered far and wide, Iran has endured invasions too, by Greeks, Arabs, Turks, and Mongols. Iran has continually reasserted its national identity throughout the centuries and has developed as a distinct political and cultural entity.

Iran is home to one of the world's oldest continuous major civilizations, with historical and urban settlements dating back to 4000 BC. The Medes unified Iran as a nation and empire in 625 BC. The Achaemenid Empire (550–330 BC) was the first of the Persian empires to rule from the Balkans to North Africa and also Central Asia from their capital in Persis (Persepolis). They were succeeded by the Seleucid Empire, Parthians and Sasanians who governed Iran for almost 1,000 years, and would put Iran once again as the leading powers in the world, only this time amongst their arch rival, the Roman Empire and the successive Byzantine Empire.

The Islamic conquest of Persia (633–656) ended the Sasanians and was a turning point in Iranian history. Islamicization in Iran took place during 8th to 10th century and led to the eventual decline of the Zoroastrian religion in Persia. However, the achievements of the previous Persian civilizations were not lost, but were to a great extent absorbed by the new Islamic policy and civilization.

After centuries of foreign occupation and short-lived native dynasties, Iran was once again reunified as an independent state in 1501 by the Safavid dynasty which established Shi'a Islam as the official religion of their empire, marking one of the most important turning points in the history of Islam. Functioning again as a leading power, this time amongst their Ottoman arch rival for centuries, Iran had been a monarchy ruled by a shah, or emperor, almost without interruption from 1501 until the 1979 Iranian revolution, when Iran officially became an Islamic Republic on 1 April 1979.

A chronology of key events:

Iran before Islam:

550-330 BC - Achaemenid dynasty rules the first Persian Empire. At its greatest extent under Darius I stretches from the Aegean Sea and Libya to the Indus Valley.

492-479 - Persian attempts to conquer Greece fail.

330 - Alexander the Great of Macedon conquers the Persian Empire, founding a short-lived empire before dying in Babylon in 323.

312-140 - Most of Persia is part of the Greek-dominated (Hellenistic) Seleucid Empire, founded by a general of Alexander the Great.

140 BC - 224 AD - Persia - known as the Parthian Empire - under the rule of the Arsacid dynasty.

224-651 AD - Sassanid dynasty rules Persian Empire; Zoroastrianism is the dominant religion.

Advent of Islam

636 - Arab invasion brings end of Sassanid dynasty and start of Islamic rule.

9th century - Emergence of modern Persian language (or Farsi), written using a form of Arabic script.

9th-13th century - Decline of Islamic Caliphate, which is replaced by a series of Iranian and Turkic dynasties, including the Shia Buyids, the Seljuk Turks and the Empire of Khwarezm.

1220 - Mongol forces of Genghis Khan overrun Persia, which becomes part of the Ilkhanate, ruled by descendants of Genghis' grandson Hulagu.

15th century - Competing Iranian, Turkic and Mongol dynasties, including the empire of Timur the Lame (Tamerlane) in eastern Iran.

1501 - With the support of Shia Qizilbash warrior tribes, Shah Ismail I becomes first ruler of Islamic Safavid dynasty; Shia Islam declared state religion.

1571-1629 Apogee of the Safavid Empire under Shah Abbas I, who reforms the army, sidelines the Qizilbash and establishes first diplomatic links with western Europe.

1639 - Treaty of Qasr-e Shirin (or Treaty of Zuhab) ends about 150 years of war against Ottoman Empire.

1736 - Nadir Shah deposes the last Safavid ruler and founds the short-lived Afsharid dynasty.

1751 - Karim Khan, of the Zand dynasty, briefly restores stability.

1794 - Mohammad Khan Qajar kills the last Zand shah and founds the Qajar dynasty, restoring stability to Iran after half a century of instability.

1828 - Iran cedes control of Caucasus to Russia after second Russo-Persian war.

1890 - "Tobacco Riots": ruler Naser al-Din Shah forced to withdraw trade concessions granted to Britain after mass protests.

1907 - Introduction of constitution which limits the absolutist powers of rulers.

1914-1918 - Iran declares neutrality but is scene of heavy fighting during World War I.

1921 February - Military commander Reza Khan seizes power.

1923 - Reza Khan becomes prime minister.

1925 December - Parliament votes to make Reza Khan ruler, deposing Ahmad Shah Qajar.

1926 April - Reza Khan crowned Reza Shah Pahlavi. Mohammad Reza, the Shah's eldest son, is proclaimed Crown Prince.

1935 - Formerly known as Persia, Iran is adopted as the country's official name.

1979 Revolution

1941 - The Shah's pro-Axis allegiance in World War II leads to the Anglo-Russian occupation of Iran and the deposition of the Shah in favour of his son, Mohammad Reza Pahlavi.

1950 - Ali Razmara becomes prime minister and is assassinated less than nine months later. He is succeeded by the nationalist, Mohammad Mossadeq.

1951 April - Parliament votes to nationalise the oil industry, which is dominated by the British-owned Anglo-Iranian Oil Company. Britain imposes an embargo and a blockade, halting oil exports and hitting the economy. A power struggle between the Shah and Mossadeq ensues and the Shah flees the country in August 1953.

1953 August - Mossadeq is overthrown in a coup engineered by the British and American intelligence services. General Fazlollah Zahedi is proclaimed as prime minister and the Shah returns.

1963 January - The Shah embarks on a campaign to modernise and westernise the country. He launches the 'White Revolution', a programme of land reform and social and economic modernisation. During the late 1960's the Shah became increasingly dependent on the secret police (SAVAK) in controlling those opposition movements critical of his reforms.

1978 September - The Shah's policies alienate the clergy and his authoritarian rule leads to riots, strikes and mass demonstrations. Martial law is imposed.

1979 January - As the political situation deteriorates, the Shah and his family are forced into exile.

1979 November - Islamic militants take 52 Americans hostage inside the US embassy in Tehran. They demand the extradition of the Shah, in the US at the time for medical treatment, to face trial in Iran.

1979 1 February - The Islamic fundamentalist, Ayatollah Ruhollah Khomeini, returns to Iran following 14 years of exile in Iraq and France for opposing the regime.

1979 1 April - The Islamic Republic of Iran is proclaimed following a referendum. The 1979 revolution ended the Shah's increasingly autocratic rule and ushered in the Islamic Republic