Eteonicus was a Spartan commander during the Peloponesian and Corinthian Wars. He participated in many key engagements, held important commands and is mentioned multiple times by Thucydides, Xenophon and Diodorus Siculus. His appearance in the record however, is mostly episodic with his roles not being particularly influential.
Eteonicus first appears on the historical stage when serving under Astyochus in a campaign around Chios sometime during 413 BC – 412 BC without any notable accomplishments.
In 412 BC – 411 BC he served as a Spartan governor of Thasos (and potentially Iasos) from where he was expelled along with the pro-Spartan faction. The setback was thought to be the result of treachery on the part of a fellow Spartan, Pasippidas, who was subsequently exiled from Sparta.
In 406 BC Spartan forces were blockading the Athenian admiral Conon in Mitylene prompting Athens to send a relief force. The Spartan admiral Callicratidas left Eteonicus blockading Conon with 50 ships and took the remaining 120 ships against the Athenians. The two fleets met at Arginusae just off the Island of Lesbos. The Spartan fleet was completely destroyed and Callicratidas killed. Eteonicus received the news of the defeat and withdrew to Chios. In the meantime, the Athenians divided their fleet after Arginusae. The main force was sent after Eteonicus while a smaller force was dispatched to pick up the survivors of the 25 ships lost in the battle. However, a bad storm prevented both endeavours. Eteonicus got away and 1000 – 5000 shipwrecked Athenian soldiers drowned leading to a political fire-storm in Athens.
Eteonicus experienced further difficulties while in Chios. As winter approached and he had no money to pay his troops they were unable to provide sufficient food and shelter for themselves. This led to a conspiracy aimed at sacking Chios which was an allied state. The conspirators agreed to tie reeds to their arms in order to recognize each other. Eteonicus feared the attack would result in significant damage to Sparta's reputation as well as damage the broader Spartan alliance. He was forced to issue the risky order to execute any of his own men seen with a reed tied around his arm. The gambit worked and the crisis was averted. Soon thereafter, Lysander arrived in Ephesus to take overall command in Ionia and Eteonicus joined him there with his forces.