During the flourishing period of Greek history, it was regarded as the chief city of the eastern Locrians, for the distinction between the Opuntii and Epicnemidii is not made either by Herodotus, Thucydides, or Polybius. Even Strabo, from whom the distinction is chiefly derived, in one place describes Opus as the capital of the Epicnemidii; and the same is confirmed by Pliny and Stephanus of Byzantium. The Opuntii joined Leonidas with all their forces at the Battle of Thermopylae, and sent seven ships to the Greek fleet at Battle of Artemisium (480 BCE). Subsequently they belonged to the anti-Athenian party in Greece. Accordingly, after the conquest of Boeotia by the Athenians, which followed the Battle of Oenophyta, in 456 BCE, the Athenians carried off 100 of the richest Opuntians as hostages. In the Peloponnesian War the Opuntian privateers annoyed the Athenian trade, and it was in order to check them that the Athenians fortified the small island of Atalanta off the Opuntian coast. In the war between Antigonus and Cassander, Opus espoused the cause of the latter, and was therefore besieged by Ptolemy, the general of Antigonus. In 198 BCE, during the Second Macedonian War they went over to the Romans.