Because Iran literally means "the land of Aryans", most historians begin their description of Iranian history with Aryans' migration. Actually the Iranian plateau was inhabited by various peoples, whose highly developed civilizations unquestionably influenced by invading Aryans. The first traces of man on the Iranian plateau belongs to 100,000 B.C, well-documented evidence of human habitation is, however, found in deposits from several excavated caves and rock-shelter sites which are dating from Middle Paleolithic or Mousterian times (100,000 B.C). Since ancient times, lowland Khuzestan has proved a favorable site for human settlements in the late 4th and early 3rd millennia B.C. At this time, it became home to the most powerful and longest-lived civilization in Iran prior to the arrival of Aryans - that of Elam. Elamite history is usually divided into three main phases: the old, Middle, and Ne-Elamite periods. In all three periods, Elam was closely involved with Sumer, Babylonia, and Assyria, sometimes through peaceful trade, but more often through war. During the Neo-Elamite period, the country appears to have been divided into separate principalities. This time is marked by constant external pressure from Assyria and Babylonia. Additionally, by 850 B.C. Small tribal groups of Aryan stock, including Persians and the Medes.
The Aryans or Indo-European, originated in the steppes of Central Asia. Around 4000—3000 B.C in an attempt to escape from the cold and from hostile neighbors, and to deal with the pressures of over-population and overgrazing in their home areas, the Aryans started to migrate toward the south and west, those who moved south became known as Indo-Iranians.
indo-european branches map
Iranian appears to have come to the Iranian plateau by routes through the Caucasian Mountains and along the Caspian Sea. Having started as mercenaries for the local chiefs, they gradually mixed with the natives and finally occupied the position of their supervisors. Among the Iranian tribes, two major groups are identifiable: The Medes and the Persians. Median Empire (728-550 B.C) has been started with the rule of Diaku who united the scattered Median clans, and organized them under a central governorship with the capital in Hegmataneh/Ecbatana. And finally this kingdom was overthrown by the sweeping new power of a group called Achaemenes (550-330 B.C).
The Achaemenes have left us no literary history of their own. What is known to us is that the Achaemenes rose to eminence in the time of Kuroush II (Cyrus II) who belonged to the Pasargadae clan of the Persians, and came of a royal line. The most prominent Achaemenid king was a leading army general named Daryush (Darius).Darius I proved to be another "Great" of the Achaemenid dynasty. Darius particularly wished to be remembered as the great lawgiver, and a law reform, with the codification of laws and the creation of a universal legal system. The efficiency of Achaemenid administration was facilitated by their famous road system, the most impressive stretch being the stone-paved Royal Road, 2,703 km long, running from Susa to Sardison the Aegean Sea. The Achaemenid Empire was the beginning of Iranian nation and history, and still constitutes one of the most glorious chapters of Iranian past.
Then Alexander conquered all of Persian Empire and he founded the Hellenistic dynasty in Iran. Alexander hoped for the fruitful union of Europeans with the peoples of Middle East. In his effort to reach this goal, he encouraged the massive settlement of Greek and Macedonian soldiers in Mesopotamia and Iran. Trying to establish strong bonds with the Iranian nobility, Alexander married Roxana, daughter of the most powerful of the Bactrian chiefs, and required 80 of his officers and 10,000 of his soldiers to marry Persian women in a mass wedding in Susa. However, Alexander's plans to bring about the union of the Greek and Iranian peoples ended when he was struck with fever and died in Babylon.
In 247 B.C Parthian Empire ruled in Iran. Parthava (Parthia) was a territory southeast of the Caspian Sea, inhabited by Parthians, a semi-nomadic people of Aryan stock. It had been annexed to the Achaemenid Empire during rule of Cyrus the Great, and had remained an Achaemenid satrapy until Alexander's invasion. Two brothers, Arsaces and Tiridates led an uprising around 250 B.C. and overthrew the Seleucid ruler. Arsaces was then proclaimed the first king, and his name became the honorific title used by all subsequent Parthian monarchs, who were generally known as the Arsacid. By 200 B.C The Parthians were firmly established on the northeast of Iranian plateau. In 224 A.D the last Parthian king, Artabanus V was killed in a battle against his former vassal in Fars, Ardashir Babakan, and the throne of Iran passed into the hands of the Sasanians. Despite the Parthians' unhappy end, their regime was the lengthiest one in Iranian history - almost five centuries.
The Sassanid dynasty rose to power as a result of the successful struggle of Ardashir Babakan, not only against his Parthian overlord, but also against a multitude of neighboring rulers. Ardashir was a son of Papak/Babak, a local king of Persis (Fars) who was a descendant of Sasan, landlord and priest of Temple of Anahita in Estakhr. The Sassanid dynasty was named for sasan in the same manner as the Achaemenid was named for Achaemenes and the Arsacid for Arsaces. The Sasanian Empire reestablished the Iranians as a superpower. For the next four hundred years, it would be recognized as one of the leading world powers in late antiquity.
Map of the Sassanid, Byzantine and Muslim borders
The Sasanian Empire at its greatest extent c. 620, under Khosrow II
By the Arabs invasion, the history of ancient Iran came to an end and the Islamic dynasties was started. The Arabs, who put an end to the Sasanians' rule, were in large part impelled by their missionary zeal for the spreading of a new religion, Islam, rather than by a desire for mere conquest. This invasion led to the decline of the Zoroastrian religion and Iranians have become Muslim.
Arab Invasion & Islamic Empires