The battle of Spartolus of 429 BC was a costly Athenian defeat in a battle fought just outside the city of Spartolus in Chalcidice.
Spartolus is in the western part of the Chalcidice peninsula, a short distance to the north-west of Potidaia. Early in the Great Peloponnesian War most of Chalcidice had revolted against Athenian control. The Chalcidians had abandoned many of their coastal communities and had adopted a new capital at Olynthus, about an equal distance to the north-east of Potidaia. The city of Potidaia itself was originally part of the Athenian Empire, but at the start of the war it too rose in revolt and was the target of a lengthy siege that only ended during the winter of 430/29 BC.
In the summer of 429 BC an Athenian army of 2,000 hoplites and 200 cavalry, under the command of Xenophon son of Euripides, invaded Chalcidice and marched up to Spartolus. The city was defended by a force of hoplites and auxiliary troops that had been summoned from Olynthus, as well as some peltasts (light infantry) from Chusis, the area to the north-west of the city, and some light troops from Spartolus.
When the Athenians arrived outside the city the defenders sallied out to meet them. The Athenian hoplites defeated the hoplites and auxiliaries from Olynthus, who were forced to retreat back into the city. Elsewhere the local cavalry and light troops defeated their Athenian equivalents, but this first phase of the battle ended with the Athenians still holding their ground close to the city.
According to Thucydides more peltasts arrived from Olynthus just after the end of this first engagement. Encouraged by the arrival of the new troops the light troops from Spartolus launched a new attack on the Athenians. The Athenians close to the city were forced to retreat back towards the troops who had been left to guard their baggage. The resulting battle demonstrated one potential weakness of a hoplite army. Every time the Athenian hoplites attempted to charge their opponents the lighter peltasts melted away, and the moment the Athenians began to retreat they were subjected to a hail of javelins, as well as repeated cavalry charges. Eventually the Athenian formations broke, and the survivors fled in some disorder back to Potidaea. The Athenians suffered very heavily casualties during this battle, with 430 men and all of the generals killed.