Obviously, the Eretrians could not forever remain in conflict with neighbors as close as the Athenians, who were so influential economically and culturally. Reconciliation came ten years after the end of the Peloponnesian War (431-404), when the domination of Sparta became much too heavy to bear for its former allies: in 394 Athens and Eretria signed, according to an (unfortunately mutilated) inscription, what was apparently the last "hundred year peace" in Greek history (subsequently, treaties were usually supposed to be valid in perpetuity). But it almost goes without saying that this alliance had its ups and downs over the following decades.
While the Eretrians (like their neighbors in Chalkis and Karystos) hastened to support Athens's efforts to form a new maritime league in 378-377, the resumption of Athenian imperialism after 375 soon bothered them, especially when Athens, which had lost Oropos in 411 because of the Eretrians themselves, succeeded in regaining control over this place of such crucial importance for relations between Attica and Euboea (371).
Having then seceded from Athens in order to form an alliance with the Thebans, who were at that time in a strong position, the Euboeans made a first attempt to constitute a confederation of cities (if not a genuine federal state), as is shown by several coins that were probably issued during the first half of the 4th century.
But starting in 366, when Eretria, with the support of Thebes, once again seized control of Oropos, the city's situation began to become critical. Its troubles were both internal, with the appearance of extremely severe social tensions -reflected in a series of coups d'état that continued almost without interruption for a quarter of a century- and international, all of Greece being at that time prey to endless conflicts.
Thanks to the speeches of Demosthenes and his rival Aeschines, the two greatest Athenian orators at the time when the city was threatened by King Philip of Macedonia, we understand fairly well the vicissitudes into which the "poor, unhappy Eretrians" were cast during the whole decade 350-340. They were tossed back and forth between oligarchs and democrats whose leaders sought in reality only authoritarian power, such as the tyrants Ploutarchos and Kleitarchos. Only in 341, when the latter was overthrown and killed as a result of an Athenian military operation, were the Eretrians able to re-establish democracy on a lasting basis. They then promulgated a law against tyranny and oligarchy and, at the same time, instituted a musical competition in honor of their tutelary divinity, the great Artemis Amarysia.
In 338, following the defeat suffered at Chaeronea by the Greek coalition (which included the Euboean cities federated in a new koinon), most of the states in Greece fell under the domination of the kings of Macedonia, first Philip and then, after 336, his son Alexander the Great. But this is not to say that from then on a city of moderate stature such as Eretria had no maneuvering room on the political level.
Particular interests continued to have full importance, as is clearly shown by the fact that, when the death of Alexander in Babylon was announced (323), the Eretrians refused to support Athens and many other cities in Greece in their rebellion against Macedonian domination. The reason was that they hoped that if the revolt was put down, their neighbors the Athenians would be deprived of the territory of Oropos that they had received from the young king of Macedonia in 335, just before his departure for the East. And that is just what happened, to the great satisfaction of the Eretrians, who were always concerned to maintain across the strait an Oropia free from political and economic domination by Athens.
by Denis Knoepfler