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

By the 6th cent BC the early Persians were established in the present-day region of Fars and were benefiting from the decline of Elam. Fars (Persis to the Greeks) was a recognizable district of the Assyrian Empire like the neighboring but larger Media. The Persian rulers, claiming descent from one Achaemenes, were associated with the Medes, who created a strong state in the 7th century. Cyaxares, son of Phraortes, the founder of Median power, was …

…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. Word Count 1,933