In the fifth century B.C. Athens formed a system of relationships with a number of other Greek city states which is often spoken of by historians as the Athenian Empire. But all the other city states retained their own governments. One "new fact" added by the Athenian Empire was the complete and effective suppression of piracy; another was the institution of a sort of international law. The law, indeed, was Athenian law; but actions could now be brought and justice administered between citizens of the different states of the League, which of course had not been possible before. The Athenian Empire had really developed out of a league of mutual defence against Persia; its seat had originally been in the island of Delos, and the allies had contributed to a common treasure at Delos; the treasure of Delos was carried off to Athens because it was exposed to a possible Persian raid. Then one city after another offered a monetary contribution instead of military service, with the result that in the end Athens was doing almost all the work and receiving almost all the money. She was supported by one or two of the larger islands. The "League" in this way became gradually an "Empire," but the citizens of the allied states remained, except where there were special treaties of intermarriage and the like, practically foreigners to one another. And it was chiefly the poorer citizens of Athens who sustained this empire by their most vigorous and incessant personal service. Every citizen was liable to military service at home or abroad between the ages of eighteen and sixty, sometimes on purely Athenian affairs and sometimes in defence of the cities of the Empire whose citizens had bought themselves off. There was probably no single man over twenty-five in the Athenian Assembly who had not served in several campaigns in different parts of the Mediterranean or Black Sea, and who did not expect to serve again. Modern imperialism is denounced by its opponents as the exploitation of the world by the rich; Athenian imperialism was the exploitation of the world by the poorer citizens of Athens.
Another difference from modern conditions, due to the small size of the Greek city states, was that in a democracy every citizen had the right to attend and speak and vote in the popular assembly. For most cities this meant a gathering of only a few hundred people; the greatest had no more than some thousands