What factors explain the Persian empire's survival for two hundred years?

What factors explain the Persian (Archaemenid) empire's survival for two hundred years? Until the sixth century BC, they were a people shrouded in mystery. Living in the area east of the Mesopotamian region, the Persians were a disparate group of Indo-European tribes, some nomadic, some settled, that were developing their own culture and religion unique from that of the great cities to their west. Sometimes history is about ideas, and nothing more clearly emphasizes this …

…as even when Greeks were taking territories the Persian gold was being poured into Greek city-states in order for them still to have influence. By 330 BC the Persians had served their purpose in reorganising the Near East. However this would have not been achieved with half of the effectiveness it was without the factors looked at in this essay which allowed the Persian Empire the time to change the shape of the Near Middle East.