The battle of Idomene (426 BC) was a second victory in three days won by Demosthenes against the Ambraciots in the north-west of Greece (Great Peloponnesian War). In the autumn of 426 BC a combined Ambraciot and Peloponnesian army had attempted to capture the city of Amphilochian Argos at the south-eastern end of the Gulf of Ambracia. This army had suffered a costly defeat at Olpae, just to the north of Amphilochian Argos. In the aftermath of this victory the Athenian commander Demosthenes discovered that reinforcements were moving south from the city of Ambracia. He dispatched an advance party up the coast to block them. One of these parties camped on the higher of the two hills of Idomene.
On the day after the battle of Olpae the Ambraciot reinforcements also reached Idomene, and camped on the lower of the two hills. That evening Demosthenes led his men (mostly Acarnanians and Amphilochians with a small number of Messenians and Athenians) on a night march. At dawn on the morning of the second day after the battle his force launched a surprise attack on the Ambraciot camp. The Messenians were put at the front of the allied army and were instructed to speak in the Doric dialect (the same dialect as spoken by the Ambraciots). The trick apparently worked, for the Ambraciots were caught entirely by surprise. Large numbers of them were killed in their camp, and many of the rest as they attempted to escape through the unfamiliar mountains. The Ambraciots lost around 1,000 men in this disaster, and Thucydides described the two defeats as the most costly to be suffered by any Greek city in such a short period of time.
Despite suffering these crushing defeats Ambracia was not occupied by the victorious allies, apparently because the Acarnanians and Amphilochians didn't entirely trust the Athenians to be good neighbours. Instead a peace treaty was agreed between the Acarnanians, Amphilochians and Ambraciots, in which they agreed to support each other against any invasions as long as the Athenians or Peloponnesians were not involved.