For the city of Eretria, the year 411 was one of the most turbulent in its history. In the spring, the Eretrians, aided by the Boeotians, who were enemies of Athens, succeeded in liberating Oropos. Until this point, control of Oropos had allowed the Athenians to dominate all of Euboea and to force the island's merchandise to pass through that point of exchange. Consequently, the Eretrians had reason to hope that the Lacedaemonians, whom they had long been urging to intervene, would finally find it possible to release all of Euboea from Athenian imperial control.
The opportunity to do so presented itself toward the end of the summer. The oligarchic government known as the Four Hundred -established in Athens after a coup d'état that had occurred a few months earlier- was definitively discredited by the arrival of a large Peloponnesian fleet off Attica. Seeing this fleet finally heading for Sounion and towards Euboea, the Athenians hastened to send a squadron to Eretria to prevent the Lacedaemonians from causing the island-which according to Thucydides was "all for Athens" -to go over to their camp (VIII 95). But the Eretrians turned out to be in league with the Lacedaemonians: when the latters' fleet, slowly sailing up the east coast of Attica, came to moor at Oropos, they waited until the Athenians (who had to supply themselves) had moved away from the shore; then, by means of a signal fire, they informed the Spartan admiral Agesandridas that the time had come to cross the strait in order take the Athenian squadron by surprise in the harbor of Eretria itself. Having hastily embarked, the Athenians were decisively defeated at sea; those among them who had hoped to find aid and succor among the Eretrians were cut down without pity as they came ashore. The only ones who escaped were those who had the presence of mind to head for the fort that Athens had in Eretria (very certainly located on the Pezonisi peninsula) and to take refuge there. Almost all of Euboea thereupon seceded.