History of Iran has been intertwined with the history of a larger historical region, Greater Iran, which consists of the area from the Euphrates in the west to the Indus River and Jaxartes in the east and from the Caucasus, Caspian Sea, and Aral Sea in the north to the Persian Gulf and the Gulf of Oman in the south.
The southwestern part of the Iranian plateau participated in the wider Ancient Near East with Elam, from the Early Bronze Age. The Persian Empire proper begins in the Iron Age, following the influx of Iranian peoples which gave rise to the Median, Achaemenid, the Parthians, the Sassanid dynasties during classical antiquity.
Once a major empire of superpower proportions, Persia as it had long been called, has been overrun frequently and has had its territory altered throughout the centuries. Invaded and occupied by Greeks, Arabs, Turks, Mongols, and others—and often caught up in the affairs of larger powers—Persia has always reasserted its national identity 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 Iranian empires to rule in Middle east and central Asia. They were succeeded by the Seleucid Empire, Parthians and Sassanids which governed Iran for almost 1,000 years.
The Islamic conquest of Persia (633–656) and the end of the Sassanid Empire 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 polity 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 who established Shi'a Islam as the official religion of their empire, marking one of the most important turning points in the history of Islam. 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.