There was a people known as the Medes who settled in the mountains east of Assyria (which was located in modern-day northern Iraq). The Medes were a tribal people related to the Persians, so we can surmise they were part of the Indo-European migration south. The Assyrian kings were infamous warmongers who went on campaigns into the surrounding lands every year to expand their territories, and they attacked and subjugating some of the Medes chieftains during these campaigns. They sowed the seeds of their own demise by doing this because it would be the Medes who would destroy them around 610 BCE when they copied Assyrian behavior.
Here is the story of the rise of the first Medes king named Deiokos told by Greek historian, Herodotus (484– 425 BCE):
There was among the Medes a wise man, named Deiokos, son of Phraortes. This Deiokos longed for power and set about it in the following way. At the time, the Medes lived in villages, and in his own village he was already a man of note; now he began to devote himself ever more keenly to the practice of justice. He did this at a time when there was much lawlessness throughout Median territory, well aware that injustice is the enemy of law. When the Medes in his own village saw how he behaved, they chose him to be their judge. And because he was motivated by his desire to gain power, he was straight and just. Because in this way he obtained great praise from his fellow citizens - so much so, in fact, that those living in the other villages were convinced that Deiokos was the only man who dispensed justice honestly. And as they were suffering from unjust decisions, they gladly went to Deiokos, after they heard about him, in order to submit their cases to him and, eventually, entrusted them to no one else.
As the number of those addressing themselves to him grew constantly (because it became known that his penalties were fair), Deiokos realized that everyone depended on him. So he refused to sit in judgement anymore and give verdicts, because it was not to his advantage to neglect his own affairs and pronounce justice every day for everybody. Soon there was more robbery and lawlessness in the villages than there had been before. Then the Medes came together and consulted about the best course of action in the present circumstances (I suppose those who spoke most were Deiokos’ friends), and said: “If the present situation continues, we shall no longer be able to live in our land, so let us appoint a king. Then peace and order will prevail, and we will be able to devote ourselves to our regular activities and not be forced to leave because of the current disorder.” These were their arguments in favor of the introduction of kingship.
As soon as proposals were made about who should be appointed king, Deiokos was put forward and praised by everybody, so that they agreed that he should be their king. Deiokos then ordered them to build a palace for him in keeping with the dignity of kingship. and to give him a guard of spearbearers. The Medes did that; they built him a large, fortified palace on the site he indicated, and let him choose his spearbearers from among all the Medes. When he had obtained power in this way, he forced the Medes to build one town and defend it in particular at the expense of the others. The Medes obeyed him in this as well, and he built large and strong walls; this now called Ecbatana. These walls are so constructed that each surrounding wall is higher than the preceding one by the height of the battlements. The site was helpful, since it was on a hill, which helped to produce this effect, but there was much planning involved as well. There are seven circles, the inner one containing the palace and treasuries. The longest wall is comparable in circumference to the one at Athens. Of the first circle, the battlements are white, of the second black, of the third purple, of the fourth blue, of the fifth orange. That is how the battlements of all these encircling walls are painted with colours. The last two ones, however, have their battlements plated, one with silver, one with gold.
This was built by Deiokos for himself and around his own house, but he ordered the rest of the people to live outside the wall. After the building was finished, he instituted a new protocol. No one was allowed to have free access to the king, instead messengers should deal with everything. The king was visible to no one; moreover it was improper for anyone to laugh or spit in his presence. He made himself exceptional so that his contemporaries, who had grown up with him and were his equals in birth and courage, would not resent his position but, as a result of not seeing him, would begin to think that he was a creature different from themselves.
By instituting these rules, he had acquired a strong position, and he showed himself a strict and impartial judge. Anyone who had a complaint had to deliver it to him in writing; he investigated in incoming complaints and made his decisions public. That is how he dealt with lawsuits. But he also introduced the following practice: when he heard that someone had committed an act of violence, he sent for him and punished him in accordance with the offence, and in the whole country he governed, he had spies and eavesdroppers in his service.
Deiokos, then, united the Median people, and ruled just them. The Median tribes were these: Busae, Paraetaceni, Struchates, Arizanti, Budii, Magi. So these are the Median tribes. (Kuhrt, The Persian Empire, p.34-35)
While Herodotus was often more a storyteller than an historian, we can see that these story elements existed contemporary to the Buddha’s time, making it difficult to assume that the Buddhist wheel-turning king stories are later stories about Asoka. The wheel-turning king legends may well be EBTs that combine various elements of legends about Mesopotamian and Persian kings. It would make sense that they may have been initially inspired by Cyrus the Great, who conquered and succeeded in stabilizing the first regional empire under the Achaemenid dynasty of Persia during the late 500s BCE. Those events would have been contemporary with early Buddhism.