Battle of Thermopylae ( 279 BC ) The Battle of Thermopylae was fought in 279 BC between invading Gallic tribes and a combined army of Greek Aetolians , Boeotians , Athenians , Phocians at Thermopylae . The Gauls under Brennus were victorious , and advanced further into the Greek peninsula where they sacked Delphi . 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 toward southern Balkans and against the kingdom of Macedonia , favoured by the state of confusion ensuing from the intricated succession to Alexander . 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 , coming from Pannonia and split into three divisions , marched South in a " great expedition " 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 Paionia ns while a third division , headed by Bolgios , aimed for 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 division , 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 quarters 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 joined the defense en masse - the old and women joining the fight . Realizing that the Gallic sword was dangerous only at close quarters , the Aetolians resorted to skirmishing tactics . According to Pausanias , only half the number that had set out for Aetolia returned . Eventually Brennus found a way around the pass at Thermopylae but the Greeks escaped by sea . Aftermath . Brennus pushed on to Delphi where he was defeated and forced to retreat , after which he died of wounds sustained in the battle . His army fell back to the river Spercheios where it was routed by the Thessalians and Malians . Some of the survivors of the Greek campaign , led by Comontoris ( one of Brennus ' generals ) settled in Thrace , founding a short - lived city - state named Tyle . Another group of Gauls , who split off from Brennus ' army in 281 , were transported over to Asia Minor by Nicomedes I to help him defeat his brother and secure the throne of Bithynia . They eventually settled in the region that came to be named after them as Galatia . They were defeated by Antiochus I , and as a result , they were confined to barren highlands in the center of Anatolia .