Hellenistic Period (323-141 B.C.)

In his world-conquering campaign, Alexander hoped for the fruitful union of the Europeans with the people of the 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 top 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. His death signaled the beginning of an internecine war among the Macedonian generals for control of his enormous empire. Alexander's widow and son were assassinated, and all but one of the officers wed that day in Susa rejected their wives. Finally, three generals won out over the others and divided the Macedonian Empire among them. Iran fell to Seleucus, of all Alexander's officers the only one who had kept his Iranian wife. He eventually became known as Seleucus I Nicator (“the Conqueror"), the founder of the Seleucid Empire,  which he gradually extended to the boundaries of the former  Achaemenid state.
Despite their far-flung plans and high-flying ambitions, neither Seleucus himself nor his successors had the competence to control their vast domain. Even before they could consolidate their power inside the country, their outlying eastern provinces had begun to revolt. The Seleucids' attention to the western territories while neglecting their eastern possessions was a political error that finally led to the decline of the Seleucid Empire. Another menace, which they could never ward off with total success, came from the warlike nomads who inhabited the immense territories beyond the country's northern frontiers and fought constantly with the settled population. Although often successful in suppressing the localized revolts of regional rulers, the Seleucid monarchs could not stem the overall tide of rebellion that arose in the country. Despite the Seleucids' strenuous efforts to introduce Greek culture in Iran, the Greeks remained alien to the Iranian people. While the westernized elite fell in with the Greek way of life, mostly in its exterior forms, the majority of the people retained their own traditional bent. After approximately a century and a half of Greek rule in Iran, the Seleucids lost all their territories east of the Eu phrates to a dynasty known as the Parthians (Arsacids), and had to content themselves with what was left - a small state in Syria and eastern Cilicia (Mediterranean coastline of present-day Turkey).