The most remarkable aspect of the Delian League is that it was a maritime empire. Earlier Greek (con)federations of Greek towns had all been land-based. A maritime empire demands another kind of organization, not in the least because the lines of communication can be threatened in the winter, whereas transport between the member states is much cheaper. This makes it unlikely that a Greek league was the model of the Athenian empire, and it is possible that the western part of the Achaemenid empire -with its maritime lines of communication and active navy- was the real source of inspiration.
The maritime organization of the western part of the Achaemenid empire was was a result of king Cambyses' conquest of Egypt (525 BCE), which was only possible after the building of a large imperial navy. (Without marine superiority, it was impossible for an army to cross through the Sinai desert, because any army marching to the west would be exposed to Egyptian naval actions.)
When Egypt was defeated and added to the Achaemenid empire, it was necessary to keep the navy to control the new region. Many men and lots of silver and gold were necessary for the upkeep, and the result was the monetarization of the tribute by king Darius I the Great (described in Book Three of the Histories by Herodotus of Halicarnassus). Although it was still possible to pay in kind, payments in cash were preferred.
The organization of the western Achaemenid empire was, therefore, largely based on the demands of the navy, and the Athenians copied certain aspects of this. For example, the ships of the Persian navy had a mixed crew: the rowers came from various parts of the empire. The Athenian ships were partly manned by Athenians, partly by the allies. Towns in the Achaemenid empire could pay their tribute by manning ships; the kings appreciated this type of tribute, because towns that had sent part of their manhood away, were less likely to revolt. The Athenians did the same.
But the main factor is the tribute system. After the Greeks had defeated the Persians, the Athenians took over the Persian fiscal organization of the Greek towns in Asia. After the Ionian revolt, the satrap of Lydia and Ionia, Artaphernes, had established the tribute that the Greek towns had to pay, and the Athenians did not change his system. Every four year, the Athenians and their subjects revised the tariff.
At least in theory, the subject towns could negotiate about the amount they owed to their masters, and it is tempting to link this fact to the remark by Herodotus that the Persians regarded king Darius as a merchant (kapelos) because he negotiated about everything (Histories 3.89). This is really remarkable, because a king was not supposed to make deals with his subjects about the prize of his reign.
The negotiations between the ruler -whether Persian or Athenian- suggest a voluntariness and an equality which probably did not really exist. But the illusion was kept intact in both empires.
The functions of the episcopus have already been described: every town in the Athenian empire, whatever its status, was supervised by an Athenian episcopus or overseer. This official kept an eye on the town where he resided. He controlled the payment of the tributes, was supposed to prevent insurrections and had to investigate evils and report them to the Athenian government. Usually, the episcopus was elected by the people's assembly.
The Achaemenid empire knew similar officials, who were called "the eye of the king". They were appointed by the king to inform him of what was going on in the empire, had more powers than the satraps, and were responsible for a well-defined region. They supervised the policy of the satraps and the payment of tribute, oversaw how rebellions were suppressed, and reported evils to the king.
The similarities are remarkable. The "eye" and the episcopus are responsible only to the highest authorities, they are supervisors of the local rulers, are responsible for the taxation, and are -in case of troubles- the direct link to the central government.
Their name may be similar as well. The real Persian title of the Eye is not known, but may have been spasaka ("seer"). If so, episkopos (which also has an association with "to see") is a translation that remains close to the sound of the original. However, this hypothetical.
It should be stressed that every ruler uses officials like the Eyes to know what is happening. The names of these inspectors may be different, but there are some primitive tasks that have to be executed anyway. For example, Charlemagne employed missi dominici. The Athenian government needed to send out inspectors, like all rulers had to do. Nevertheless, because the job responsibilities of the Eye and the episcopus are so very similar, we must seriously entertain the possibility that the Athenians copied a Persian function.
In the fields of architecture and politics, the Athenians of the fifth century BCE copied several Persian innovations. In the branch of architecture, this happened in two ways: practical and ideological. The first of these can be found in the production and elaborating of rhytons, but also in the building of the Odeon and the Prytaneum. A Persian tent (and therefore a Persian architectural style) was used when the city was rebuilt and offered space for cultural and political activities. At the same time, they offered proof of the Athenian victory in war.
The second type of emulation can be found in the Parthenon frieze and the caryatids. The difference is twofold: in the first place, the caryatids and the frieze are not based on something tangible like rhyta or tents; in the second place, not only a from, but also a general idea are copied. In the Parthenon frieze, the Persian ideal of "unity under the king" has been "translated" to Greece. The image and idea were adapted to Greek tastes, which made the work of art more accessible. In the caryatids, the original image (a bull or a feline) has been ignored and only the essence, the general idea, is copied - to women. Apparently, the Greeks found women better motifs to show subjection than animals.
Summing all up, a case can be made for the existence of Persian influence on Greek art. The same can be said for politics. The Athenians and Persians both were masters of the Greek towns in Ionia, and since the Athenians had no experience in ruling an empire (whereas the Persians stood in a long tradition), they copied Persian measures. Therefore, they copied the tribute system, organized their navy like their enemies did, and appointed episcopi to control the subject towns.
It was the obvious thing to do. After all, it is sound policy to make use of knowledge developed by others. Nineteenth-century European historians, however, have often ignored the Persian contribution to Greek culture. They believed in a "Greek miracle" and were unable to conceptualize oriental influences. (They had more or less the same perspective on European history, which had developed -in their view- autonomously.) Cultural contacts were ignored. Today, in a world in which cross-fertilization and clashes between cultures can no longer be ignored, scholars are more interested in cultural contacts. This perspective does more justice to the complexities that existed when two cultures encountered each other.
J. Balcer, "The Athenian episkopos and the Achaemenid King's Eye" in: American Journal of Philology 98 (1977) 252-263
John Boardman, Persia and the West (2000 London) [for a completely different interpretation of the evidence]
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J.A.S. Evans, "The settlement of Artaphrenes" in: Classical Philology 4 (1971) 344-348
E.D. Francis and Michael Vickers, Sigma priscae artes. Eretria and Siphnos, in: the Journal of Hellenic Studies 103 (1983) 49-67
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H.T. Wallinga, "Persian tribute and Delian tribute" in: Pierre Briant and C. Herrenschmidt (eds.), Le tribut dans l' Empire Perse (1989 Paris) 173-182