As a whole it is believed that the widespread migration of the Aryan tribesmen into the Iranian plateau started at the end of the second millennium B.C. Although traces of their arrival have been noted at Tappeh Hessar, near Damghan or Tappeh Silk, near Kashan, during the third millennium B.C, in all probability this was the first migration of the Aryans into the heart of the Iranian mainland. But the second migration differed from the first invasion. This was a continued wave of invasion starting from Eurasian plains south of Russia and advancing into the south from two fronts:
1. Western branch where the migrants circled the Black Sea and after crossing the Balkans and Bosphorus they penetrated the Asia Minor and formed the Hittites confederation.
2. Eastern branch known as Indo-Iranians branch which moved from east of the Caspian Sea and advanced from the following directions:
Moving from Caucasus to the great Euphrates bend, the new comers mixed with the Hurrians, Asiatic natives, and formed the Mitanni kingdom.
Moving from the Central Zagros mountains range this branch was assimilated by the dominant Cassette tribe.
Moving from eastern front and crossing the Transoxiana and Jeyhoon river through Hendukosh pass and the ancient Indian road. Probably part of these Aryan hordes turned and moved into Iran.
The reason for such exodus by the Aryan tribe was the pressure applied by the yellow-skinned tribes in the north, sudden drop of temperature, increased tribal population and drying of pastures and their quest for new and fresh pastures. During the migration, these tribesmen were confronted with natives such as Amardes, Tappuris, Caducians, Ilamates and ugly looking negroes and were forced to war with them. The description of such battles has nowadays ceased to be historical and factual and has become fable. Thus the Iranian fabulous epics such as the Shahnameh refer to such battles as wars between the Iranians and the divs. Also the wars that were fought between the Iranians and the Sogdians northeast of the country and around the Caspian Sea is also known as war with Turanians.
The first symptom of such immigration and the changes that these new tribes effected in Iran was the appearance of dark gray earthenware which did not exist in these regions before their arrival. The Aryan tribesmen entered the new regions along with their families, herds, war horses and shepherd dogs, and were gradually employed as mercenaries by native governors to defend and patrol villages. After several centuries these new tribes gradually replaced the natives in Iran. The symptoms of such new arrivals and replacement are notable in the following changes. The dead which were formerly buried in the house floors, were transferred to a distance of approximately hundred meters from the city and were buried in graveyards. Of course these people continued to bury the various equipment and belongings of the dead person under the ground along with the dead persons. The disk of the sun, mountain goat, a horse or horseman images on the tombstones and alternative use of unbaked brick and stones on the walls were indications of the Aryan community. The Aryans fabricated swords, daggers, spears and bows with bronze or iron.
A careful study of the historical hills in that period specially the graveyard in Tappeh Silk distinctly point to rich and poor classes of ancient Aryans based on the ornaments that were buried with the dead or heavy stones carrying the image of angels that covered the tombs of richer persons. On the contrary the graves of the medium and poor class lacked such ornaments. This new custom of burials which wholly differed from burial of earlier settlers, is another indication about the arrival of a new race of people in the Iranian plateau. Influenced by the local languages and religious customs caused the mixture of the Aryan (Iranian) and the native cultures, and gradually assimilated the earlier settlers in Iran. The method of the life of the Iranian shepherds was also mixed with the customs of native villagers and farmers and led to a new social life called "shepherd farmers." This rapidly contributed to the growth of the economy and wealth in the new community.
Of political features of that period one can refer to the repeated incursions by the Assyrian government into the Iranian mainland to obtain iron, horses, slaves, and precious metals and stones which were used by the natives at those times. In Tappeh Silk which had become an Aryan town, one of the governors had built a magnificent castle over the hill. This was a well fortified castle equipped with a palace, residential quarter and a graveyard. This governor built a wall that circled the castle along with watchtowers for patrolling and a moat. Of course such architecture called for flatting of the land to lay the foundation of the palace and residential quarters for nobles and the Aryans needed a group of workers such as masons, stone workers and bricklayers, etc. which were naturally performed by the tamed natives. In this new community the governors not only warred with neighboring local rival governors but were in continuous war with big western kingdoms such as the Assyrians.
An Aryan prince used to live with his courtiers and companions. He governed the farmers and his income came from his estates and war pillage. The community was divided into several categories including governors or princes, nobles, free citizens or landowners and slaves, but the peasants during this period enjoyed more liberty in Iran compared with those who lived in Mesopotamia and Egypt. Because petty landowners exited in the Iranian villages.
Besides agricultural development, the early Iranians paid due attention to extraction of mines and dug out copper, iron, and lapis azuli. Meanwhile the ruling governor supported and encouraged commerce, and ornamental gold and silverware or precious stones or fabrics were traded. As a result the economy was based on two foundations: farming and commerce. But the economy did not flourish continuously because warlike tribes such as the Assyrians which lived west of Iran, always attacked and plundered the region and caused a lot of unrest. This was because Mesopotamia and the Assyrian Empire were always in need of raw material. The Assyrians were unable to receive such minerals and resources from Asia Minor or Caucasus because that region was governed by the strong Urartu Empire. As a result they were compelled to plunder such items from their eastern frontiers and Iran which was not governed by a united central government.
Meanwhile since the Ilamite kingdom had been weakened and destroyed as a result of repeated wars with the Chaldians and Assyrians, the Assyrians were further emboldened. Eventually in a battle with Assurbanipal in the year 645 B.C. the Ilamite king was defeated and his country and Susa, his capital, were captured and plundered by the Assyrian army.
During such times the movement of the Aryan tribes within Iran had not been completed. At those times the new comers built small scattered towns mostly at the western regions and posted their cavalry to protect them. Of course the small cavalry which was formed of 200 or 300 horsemen were unable to defend the strong and warlike Assyrian army. As a result when attacked by the Assyrians the citizens and their small armies fled and sought refuge in the mountains. Thus for a long time the Aryans and the natives in the western regions and specially at the foot of Zagros Mountain were agitated and harassed by the invaders. During these incursions the Assyrian army plundered corn and goods along with horses and cattle and if they faced defenders they enslaved them and carried them back to their homes.
For example an Assyrian almanac has recorded that during the attack by Tiglathpukeser III in the year 745 B.C. against scattered Parsi and Median tribes west of Iran, the Assyrians captured 60,000 slaves and seized immense number of cows and sheep. In this almanac for the first time, the Assyrian scribes refer to Parsis as Parsuvans in the year 844 B.C. and Medes as Madays in 836 B.C. At that period the Parsis were living west and southwest of Urmia Lake and the Medes were settled southeast of the lake near Hamedan. But later obsessed by repeated Assyrian incursions and persecuted by the northern Urartu government, the Parsis moved towards the Bakhtiari mountain range and east of Shushtar and settled in a region called Parsuash or Parsumash around 700 B.C.
Therefore, due to the reasons mentioned above and the special nature of the Iranian vast plateau when towns and villages were scattered and disunited, the country was unable to resist united against enemy attacks or if they resisted such resistance was scarce and feeble. The idea to resist the Assyrian oppression by consolidation was first developed among the Aryans and specially the Medians who lived northwest of Iran and were more exposed to continued Assyrian incursions.
Until that time rivalry between rival feudal Iranian tribes, their scattered nature and lack of unity among them, served as a winning card for the aggressive Assyrians. Dayaukku, one of the leaders of the Median tribes, who had become known as a judge among tribesmen from the beginning, succeeded to unite six Median tribes with each other and form the first Aryan government in Iran around eight century B.C. He placed his seat in Hegmataneh which means the place of congregation. Thus Dayaukku succeeded to unite the scattered Median tribes and form a single nation. After the unification of the tribes and establishment of an Iranian kingdom northwest of Iran, the southern Persians laid the foundation of the glorious Achaemenid empire.