Eion (Ancient Greek: Ἠϊών; gen.: Ἠϊόνος) was an ancient Greek Eretrian colony in Thracian Macedonia. It sits at the mouth of the Strymon River which flows into the Aegean from the interior of Thrace. It is referred to in Thucydides' History of the Peloponnesian War as a place of considerable strategic importance to the Athenians during the Peloponnesian War.
Eion was occupied by the Persians in 476 BC in the aftermath of the Greco-Persian Wars. It was then captured by the Delian League in 475 BC under the leadership of the Athenian general Cimon, the son of Miltiades the Younger. He captured the city by turning the course of the River Strymon so that it flowed against the city walls, causing the mud brick fortifications to melt. The inhabitants were enslaved. The capture of Eion was the beginning of a military campaign undertaken by the newly formed Delian League, whose objective was to clear the Aegean Sea of Persian fleets and pirates in order to facilitate Athenian access to the Hellespont.
In 424 BC, during the Peloponnesian War, Eion was the site where the Athenian commander Aristides intercepted a Persian messenger named Artaphernes. The message, which was on its way to Sparta, was a letter from the Persian king addressing previous requests made to him by the Spartans.
Later in the war, in the winter of 424 BC/423 BC, the Spartan general Brasidas captured Amphipolis with his Thracian allies. When he moved against Eion, however, he was unable to overcome the Athenian defenders, who were led by Thucydides. Although he held Eion, Thucydides was subsequently ostracized by the Athenians for his failure to defend the more pivotal city of Amphipolis.
- Keith G. Walker, Archaic Eretria: A political and social history from the earliest times to 490 BC (ISBN 0-415-28552-6), p. 154.
- Mogens Herman Hansen, An Inventory of Archaic and Classical Poleis: An Investigation Conducted by the Copenhagen Polis Centre for the Danish National Research Foundation, 2005, p. 827.
- Peter Edmund Laurent, The Nine Books of the History of Herodotus, Vol. 2 (Henry Slatter, 1837), p. 415.
- Gocha R. Tsetskhladze (ed.), Greek Colonisation: An Account of Greek Colonies and Other Settlements Overseas (Brill, 2008; ISBN 9004155767), p. 67.