The battle of Apollonia (381 BC) saw Sparta's ally Derdas of Elimia defeat an Olynthian cavalry raid that had entered the territory of Apollonia.
In 382 the Spartans had received two embassies asking for help against Olynthus, a rising power in Chalcidice, and had decided to send an army north to aid Amyntas III of Macedon and the Chalcidian cities of Acanthus and Apollonia. Their first army was sent in two waves, of which only the first reached Thrace, where it soon bogged down. The second wave ended up seizing power in Thebes.
The Spartans responded by sending a second army to Thrace, this time commanded by Teleutias, the half brother of King Agesilaus II. On his way north Teleutias took care to gather allies, amongst them the Thracian king Derdas of Elimia. Soon after arriving in the area Teleutias led his army to Olynthus, where he was saved from defeat by Derdas (battle of Olynthus, 382 BC). Although Teleutias claimed this had been a victory, over the winter of 382-381 BC the Olynthians carried out a series of raids into the territory of Sparta's allies in the area.
In the spring of 381 the Olynthians sent a force of six hundred cavalry to raid Apollonia, north of Olynthus. By noon on the day of the raid the Olynthian cavalry was quite widely spread, plundering the local area. Unluckily for the Olynthians, on the same day Derdas had arrived in Apollonia with his cavalry.
Derdas waited until the raiders were approaching the city walls of Apollonia before he unleashed his own cavalry. This caught the Olynthians by surprise, disorganised, and probably separated into smaller groups in the suburbs of Apollonia. In contrast Derdas's men were well concentrated, and they quickly forced the raiders to flee. The Olynthians were pursued all the way back to their city walls, losing eighty dead during the fighting.
This was the high point for Teleutias. Later in 381 he launched another raid into Olynthian territory, but he was caught by the Olynthian cavalry, which had clearly not been too badly damaged at Apollonia, defeated, and killed (battle of Olynthus, 381 BC). This didn't end the bad news for Sparta - a fresh army, commanded by King Agesipolis, was sent north, but didn't achieve much before the king died of a fever in the summer of 380 BC.