The Persian Empire
Persians managed to create the very first Indo-European global empire which was the vastest at that time. It is said that Persians showed tolerance and respect towards all people in their empire. However, one should note that, this was mostly because they were smart enough to desire political stability which is often destroyed by ethnic clashes. They had at the same time a great appetite for conquering other nations and assimilating them into the Persian culture. In this respect they followed the style of their neighbors, the Babylonians and Assyrians. They brought stability to a Mesopotamia in which for example there was no tradition of releasing the war captives at the time.
It is a historical fact that Persians showed great tolerance towards the Jewish people who were basically prisoners in Babylonia, but one might ask the question: Would the Jews have been tolerated, had they been in the way of the Persian supremacy in the Middle East? Indeed, it is quite possible that Persians would have assimilated the Jews. That was after all what happened to the Persians' own kinsmen, the Medians.
There are many inscriptions made by the Great Kings of Persia, in which they boast about how tolerant they were towards different races, religions and backgrounds, distancing themselves from the oriental tyrants of the Middle East who kept boasting about how many people they had killed and captured in battles. But, what would Medians, Scythians, Armenians, Greeks, Babylonians and Africans (especially Egyptians) have to say about that? One may say, well, Medians and Scythians ended the Assyrian dominance in the Middle East also! True, but it was mainly because Assyrians oppressed them (and so many others). Assyrians were well-known for their ruthless and brutal hunger for power and the Median vassal king Fravartish was killed in a battle trying to keep his own territory. Much Later, the Medians rebelled several times against the Persian dominance in their land, but they were defeated every time and their leaders were executed. Thus the Medians lost any long-lasting influence in the global Iranic culture.
Finally, even the Persian Achemaenids fell to Alexander of Macedonia and Persians lost their vast empire to Greeks and Macedonians. After the Macedonian invasion, a Greek dynasty, the Seleucids was established in Iran and they in turn tried to "Hellenize" the culture. This was not considered to be a great loss by the Persian aristocracy, as the incorporation of foreign elements into their already multicultural world was normal. Eventually, and not much later, the Seleucids military weakness and the lack of political power initiated the rise of the Parthian dominance in the eastern provinces of Iran. Parthians were Scythians and belonged to the Parni, which was a branch of the Dahae confederation of Scythians. The Parthians managed to gradually reestablish an Iranic domination of the Iranian plateau. The Parthians ruled Iran for nearly 400 years. They created the first institutionalized elective system of kings (although it failed eventually) through their own parliament Mahestan. This is perhaps the first known "House of Lords" in the Indo-European history. The Parthians revived the Iranic martial ardor lost under the Seleucids, and restored the Iranic culture in Iran against the continuous Roman aggressions, but never accepted by their Persian antagonists, they eventually lost their power to the Persian Sassanids.
The Sassanids warred with Romans for almost four centuries, and later with the Byzantine Empire, and the Huns and the Turks. Most of their wars ended disastrously for the Iranian people. Outside Persia they firmly held the lower Tigris-Euphrates Valley. The Sassanids promoted and redesigned the Zoroastrian religion, punishing by death those who left the official faith. Zoroastrianism, which is by many, considered as one of the purest Iranic cultural remains, was opposed vehemently by many Iranic tribes who preferred their own Mithra and Anahita to Zoroastrian paradigm of duality. The Sassanids are also honorably responsible for other cultural contributions, such as the spectacular architecture they produced in Iran and Mesopotamia, some of which remain until this day.
In the 7th century Sassanid Persia fell to the conquering armies of Islam of Arabia. Both the Sassanids and their enemies, the Byzantines, had been seriously weakened by exhausting wars they had fought between 603-629 ACE. The Persian Sassanids, shaken by defeat, ruled by an unstable and corrupt dynasty, with a disrupted army and civil service and with a people alienated by crushing taxation and parasitic landlords, and a non-organic religion had to give in for the "vigorous impulse of Islam". Islamic rule, under the Caliphate, persisted for seven centuries. It gave the Persians a wholly new religion and altered their way of living. Yet Persian culture did not die.