Basically, the Greeks (Spartans? Byzantines?), like in OTL, became important mercenary exporters. One particularily large mercenary force was hired by the faraway king of Bactria, to fight rebels. The journey was long and ardous, but the Greek mercenaries were tough enough to endure it. They put down the rebels and decided to settle down there rather then march all the way back. The local king didn't like it, and tried to attack them when they refused to move. Superior training and disciplinne of the Greeks, in addition to the ambitiousness of their commander, allowed them to crush the Bactrian armies and to kill the king. After that, their commander declared himself king of Bactria, and though it didn't sit well with the natives, after the bloody defeat of the first two rebellions they had to live with it.