The battle of Olpae (426 BC) was an Athenian victory that ended a Spartan campaign aimed at the conquest of Acarnania and Amphilochia (Great Peloponnesian War). In the autumn of 426 BC the Spartans had sent an army of 3,000 allied hoplites to support the Aetolians in an attack on the important Athenian naval base at Naupactus on the northern shore of the Gulf of Corinth. This attack had ended in failure (siege of Naupactus) after the Athenian commander Demosthenes had moved 1,000 reinforcements into the city by sea, but instead of abandoning the campaign the Spartan commander Eurylochus had accepted a plan for an invasion of Acarnania and Amphilochia. Acarnania was the area along the coast between the Gulf of Corinth and the Gulf of Ambracia, while Amphilochia was the area around Amphilochian Argos, a city at the south-eastern end of the Gulf of Ambracia. The campaign was suggested by the Ambraciots, the inhabitants of the Corinthian colony of Ambracia, a city located seven miles to the north of the Gulf of Ambracia. The Ambraciots promised to mobilise and attack Amphilochian Argos early in the winter, and so the Spartans dismissed their Aetolian allies and took up a position at Proschium, in the south-west of Aetolia.
As promised the Ambraciots made their move in the winter of 426 BC, advancing around the eastern end of the Gulf of Ambracia with 3,000 hoplites. They occupied a position at Olpae, just under three miles to the north of Amphilochian Argos, at the eastern end of the Gulf. The Acarnanians responded to this move by moving most of their army to Amphilochian Argos. One division of the army went straight to the city, while the rest took up a position at Crenae, guarding the south-eastern approaches and the route along which the Spartans were expected to move. They also called for Demosthenes, asking him to take command of their army, and for help from the twenty Athenian triremes then cruising around the Peloponnese.
Eurylochus and his Peloponnesian army took an unexpected route to Orpae. From Proschium they advanced north into Acarnania, and advanced to the west of Stratus, the largest town in the area. They then marched to Mount Thyamus, in Agraean territory (to the south of Crenae), and from there slipped through the gap between Amphilochian Argos and Crenae and joined up with the Ambraciots at Olpae.
Soon after this Demosthenes arrived with the twenty triremes, 200 Messenian hoplites and sixty Athenian archers. He was placed in command of the combined Athenian/ Acarnanian and Amphilochian army and advanced towards Olpae, where he camped on the opposite side of a ravine from the Peloponnesian army. The two sides then remained in their camps for five days, before on the sixth day both armies came out of their camps and prepared for battle.
The Peloponnesian army was the larger of the two. It was arrayed with Eurylochus and his own troops on the far left, a Mantinean contingent next in line and then mixed contingents of Peloponnesians and Ambraciots along the rest of the line.
Demosthenes realised that his shorter line would probably be outflanked, and so set a trap for the Peloponnesians. A force of 400 hoplites and light troops (mainly Acarnanians) was hidden in bushes on an overgrown path. Demosthenes, the Messenians and Athenians formed the right of the line, facing Eurylochus. The rest of the line was made up of Acarnanians and Amphilochian javelin-throwers.
At first all went well for Eurylochus. On his right the Ambraciots defeated their opponents and drove them off the battlefield, but instead of turning back to help in the centre they pursued the defeated troops back towards Argos. On his left the Peloponnesian troops soon outflanked Demosthenes on the Acarnanian right, and began to encircle them. At this point Demosthenes sprang his ambush. The 400 hidden troops plunged into the back of the Peloponnesian left which dissolved in chaos. The panic spread along the line, and soon the entire left and centre of the Peloponnesian army was retreating in disorder (apart from the Mantineans, who managed to stay together and retreat in good order). Eurylochus was killed during the collapse of his army. At this point the victorious Ambraciot right wing returned to the battlefield to find the rest of their army fleeing in defeat. The Acarnanians attacked them as they attempted to retreat back along the coast to Olpae, and inflicted heavy casualties on the retreating troops.
The battle lasted until the evening, although we don't know how early in the day it began. On the day after the battle the new Peloponnesian commander, Menedaius, approached Demosthenes to ask to recover their dead and to ask for a truce which would allow the defeated army to retreat to safety. Demosthenes agreed to the first request, but refused to agree to a truce for the entire defeated army. Instead he made a secret agreement in which Menedaius, the Mantineans and the more important Peloponnesians were given permission to leave. He had two motives for this move - first to weaken the Ambraciot army and second to discredit the Spartans by showing that they would abandon their allies after a defeat. On the day after the battle the Peloponnesians took advantage of the secret agreement to retreat. Their Ambraciot allies attempted to join them, and in the resulting fighting another 200 were killed before most escaped east to safety into Agraea.
Two days after the victory at Olpae Demosthenes won a second major victory a short distance to the north, at Idomene. These two victories helped to restore his reputation back at Athens, and he was able to safely return to the city, taking 300 sets of enemy armour with him as a trophy. The Acarnanians and Amphilochians were unwilling to let the Athenians occupy Ambracia, and instead agreed a peace treaty in which each side promised to support the other against any invasion unless the Athenians or Peloponnesians were involved.