For 200 years, from the second half of the sixth century to the decades before 330 BC, the Persian dynasty of the Achaemenids ruled an enormous empire stretching from the Mediterranean Sea to Afghanistan and India. The Great Kings Dareios I and Xerxes I even tried to conquer Greece and the northern Black Sea territories. Although they failed, parts of Thrace did become part of their dominion for a short period. The question always rises as to why the Great Kings were interested in the western and northern Pontic zones. In contrast to some of the other satrapies, such as Egypt, Phoenicia and Syria, the Black Sea had no prosperous cities or provinces to offer. One possible answer might be the desire to conquer every part of the known world. After 479 BC, it seems that the Great Kings acknowledged the fact that the coast and the Caucasus formed the natural borders of their Empire. The satraps, on the other hand, could not avoid becoming involved in the affairs of the Black Sea region in order to safeguard the frontiers they had established. They had to incorporate the Greeks, as accepted inhabitants of their province, into the Persian administrative system. Possibly they achieved this by granting them the monopoly in sea trade and using the Anatolian Greeks as the main active bearers and transmitters of Persian customs and culture. More research into this chapter of Persian history is still required.