Dependence upon Sparta or Athens was, in fact, regarded by none of their allies except as the less of two evils: the greater was dependence upon their domestic foes. Hence the tyranny just described did not arise with the consent of the tyrannized. The allies of Athens had consented to enter only into alliance (symmachia) with her on stipulated terms and for a stipulated purpose--protection against Persia. What they had neglected to stipulate was the time for which they were to remain allies. Athens, accordingly, denied them the right to secede, and when particular cities tried none the less to withdraw, she made the preservation of the union a moral ground for coercion, and with the aid of such cities as remained faithful, and the fleet which she kept ready for action by the financial contributions of all, she forced them back on terms such as a conqueror could dictate. A new treaty of alliance was, however, the future, as it had been the ancient, tie. And speaking broadly, we may affirm that in the city-state world of classic Greece an empire was legally impossible: what we, and the ancients, looking to realities, call an empire was an aggregate knit together by treaties, the very formation of which shows that we have to do, not with a single sovereign, but with a group of sovereigns. In other words, the city remained the ultimate political unit. The rule of Athens and Sparta was, strictly speaking, an hegemony and not an _arche_; a shifting and temporary leadership, and not a permanent suzerainty. It was a necessity of circumstances assumed to be exceptional. * * * * * Unfortunately, experience showed that the circumstances in which imperialism was a political necessity recurred constantly. After the fall of Athens in 404 B.C., a defensive war against the barbarians--the Macedonians in Thessaly, the Persians in Ionia--served as a justification to Sparta in employing force to maintain the hegemony which she had won. But in 387 B.C. the peace known as the "King's Peace," or the "Peace of Antalcidas," was concluded with Persia, whereupon it became impossible to use any longer the national cause as even a pretext for tyranny. The hegemony, however, was not abandoned. It had to be maintained, it was alleged, to keep the other cities free, and to this end Persia lent aid to Sparta and Thebes successively. If an empire could only be prevented by an empire, and national recreancy to boot, the times were surely out of joint. Such an issue was the _reductio ad absurdum_ of the system of hegemonies, as both reformers and statesmen in Greece came speedily to realize.