The battle of Aegitium (426 BC) was an Athenian defeat that ended a short-lived invasion of Aetolia (Great Peloponnesian War). In 426 BC Athens sent a small fleet of 30 warships under the command of Demosthenes around the Peloponnese to operate in the north-west of Greece and the Corinthian Gulf. Once in the north-west Demosthenes was able to gather a strong allied army, with contingents from the islands of Corcya, Cephalonia and Zacynthos and from the Acarnanians on the mainland. He then began a blockade of Leucas, a Spartan ally on an isthmus close to the entrance to the Gulf of Ambracia.
This blockade never quite developed into a formal siege. Instead Demosthenes was distracted by an alternative plan put forward by the Messenians. They suggested that he invaded Aetolia, the largely mountainous area inland from the northern coast of the Gulf of Corinth. This plan appealed to Demosthenes partly because it would remove the Aetolian threat to the Athenian naval base at Naupactus, on the northern coast of the Gulf, and partly because it would allow him to invade Boeotia from the west, potentially eliminating a major threat to the Athenians. The Acarnanians were less impressed with the new plan, and refused to take part in it. This left Demosthenes with a mixed force of Cephallenians, Messenians, Zacynthians and 300 Athenians. He could also have had a contingent of light troops from Locris, but decided to begin his invasion before they arrived.
Demosthenes sailed east along the Greek coast from Leucas to Oeneon in Locris (the coastal area to the north of the Gulf of Corinth). He then marched north-west into Aetolia, taking Potidania on the first day and Crocylium on the second day before turning west to take Tichium on the third. By this point it was becoming clear that the Aetolians had not been surprised by the invasion, as Demosthenes had hoped, but were instead massing their army against him. Despite this he decided to continue with the invasion. This time the allied army marched east and captured the town of Aegitium, nine miles north of his starting point at Oeneon.
The inhabitants of Aegitium escaped from the town and joined up with the main Aetolian army. Because he not waited for the Locrians Demosthenes had very few javelin-armed troops, although he did have a contingent of archers. The Aetolian army was almost entirely made up of lightly armoured javelin-throwers, and they used their mobility to great effect, running in to attack the Athenians and their heavy troops, retreating whenever the Athenians attempted to advance or when they came under attack by the archers. The battle continued in this way for some time, with the Athenians unable to get into close combat with the Aetolians, while coming up under constant javelin attack.
The Athenians were just about able to hold their ground until the commander of the archers was killed. After this his men scattered and the Aetolians were able to press their attacks with less risk. The Athenians and their allies were now becoming increasingly tired, and eventually the allied army broke and fled. The retreat was disastrous. Demosthenes' Messenian guide Chromon had been killed, and very few of the Athenians knew the area. Large numbers were killed after they fled into dried-up water-courses, where they became trapped between the high dry banks. Others fled into a forest which was then set on fire, again killing many of them. Thucydides doesn't give casualty figures for the entire allied army, but does state that 120 of the 300 Athenian hoplites were killed, along with Procles, the co-commander of the army. The survivors of the army eventually reached safety back at Oeneon. Most of the Athenians returned home, although Demosthenes didn't dare return to the city. Instead he stayed in the west, where he soon restored his reputation with victory at Olpae.