The Battle of Thermopylae was fought in 279 BC between invading Gallic armies and a combined army of Greek Aetolians, Boeotians, Athenians, and Phocians at Thermopylae. The Gauls under Brennus were victorious, and advanced further into the Greek peninsula where they attempted to sack Delphi but were completely defeated. Background Gallic groups, originating from the various La Tène chiefdoms, began a south-eastern movement into the Balkan peninsula from the 4th century BC. Although Gallic settlements were concentrated in the western half of the Carpathian basin, there were notable incursions, and settlements, within the Balkan peninsula itself. From their new bases in northern Illyria and Pannonia, the Gallic invasions climaxed in the early 3rd century BC, with the invasion of Greece. The 279 BC invasion of Greece proper was preceded by a series of other military campaigns waged in the southern Balkans and against the kingdom of Macedonia, favoured by the state of confusion ensuing from the complex and divisive succession processes following Alexander's sudden death. The Celtic military pressure toward Greece in the southern Balkans reached its turning point in 281 BC. In 280 BC a great army, comprising about 85,000 warriors, approached from Pannonia and split into three divisions. These forces marched south to Macedon and central Greece. Under the leadership of Cerethrius, 20,000 men moved against the Thracians and Triballi. Another division, led by Brennus and Acichorius moved against Paionians, while a third division, headed by Bolgios, headed towards the Macedonians and Illyrians. Bolgios inflicted heavy losses on the Macedonians, whose young king, Ptolemy Keraunos, was captured and decapitated. However, Bolgios' contingent was repulsed by the Macedonian nobleman Sosthenes, and satisfied with the loot they had won, Bolgios' contingents turned back. Sosthenes, in turn, was attacked and defeated by Brennus and his forces, who were then free to ravage the country. After these expeditions returned home, Brennus urged and persuaded them to mount a third united expedition against central Greece, led by himself and Acichorius. Battle A Greek coalition made up of Aetolians, Boeotians, Athenians, Phocians, and other Greeks north of Corinth took up positions at the narrow pass of Thermopylae, on the east coast of central Greece. During the initial assault, Brennus' forces suffered heavy losses. Hence he decided to send a large force under Acichorius against Aetolia. The Aetolian detachment, as Brennus hoped, left Thermopylae to defend their homes. The Aetolians all joined the defence -