The battle of Erigon Valley or the Lyncus Plain (358 BC) was the first major battle during the reign of Philip II of Macedon, and saw him defeat the Illyrian king Bardyllis.
Philip came to the throne after his predecessor, Perdiccas III, was killed in a battle against Bardyllis and his Illyrians. In the aftermath of the battle the Illyrians occupied part of Upper Macedonia, but Philip was then able to arrange a temporary truce with the Illyrians. He used the breathing space this gave him to begin his famous reforms of the Macedonian army, although there is an inevitable debate about how much he achieved, and how quickly. At the same time he managed to undermine a number of rivals to the throne, and arrange a peace with the Paeonians, to the north of Macedonia.
The reformed (or reforming army) first saw combat early in 359, when the pretender Argaeus made a bid for power. Although the Athenians had officially supported Argaeus, Philip used the opportunity given by his defeat to form a new alliance with Athens.
His next target were the Paeonians, weakened by the death of their experienced king Cotys. They were quickly defeated, eliminating another threat.
Philip now turned his attention to Bardyllis. Bardyllis realised that an attack was coming, and attempted to extend the peace by offering a long term peace based on the status quo. Unsurprisingly this didn't appeal to Philip.
Philip raised a levy of 10,000 infantry and 600 cavalry for the upcoming battle. This was a much smaller army than the one he could field later in his reign, but was still a sizable force. According to Diodorus (16.4.3) Bardyllis had a very similar force (10,000 infantry and 500 cavalry).
The Illyrians formed into a square, probably with their best men in the centre. Philip probably arranged his army with his infantry in the centre and his cavalry on the flanks. He commanded on the right, where the best of his troops were placed. He was probably in command of part of the infantry, as according to Diodorus he ordered the cavalry to outflank the Illyrians, while he launched an attack on the weaker Illyrian flanks.
The Illyrian square proved to be a difficult defensive formation to overcome, but eventually the Macedonian cavalry was able to attack its sides and rear, and the Illyrian formation broke. The Macedonians pursued for some distance, before Philip recalled them. Diodorus gives the Illyrian losses as 7,000 dead but gives no Macedonian losses.
In the aftermath of the battle the Illyrians agreed to give up all of their previous conquests in Macedon, and made peace on Philip's terms. They were pushed away from the Macedonia heartlands, making Macedon less vulnerable to attack from the west.