Brennus (or Brennos) (d. 279 BC) was one of the leaders of the the army of Gauls who invaded Macedonia and northern Greece, defeated the assembled Greeks at Thermopylae, and is popularly reputed to have sacked and looted Delphi, although the ancient sources do not support this.

Brennus is said to have belonged to an otherwise unknown tribe called the Prausi.[1] These Gauls had settled in Pannonia because of population increases in Gaul, and sought further conquests. The army was initially led by Cambaules, who led them as far as Thrace, where they stopped. When they decided to advance again in 279 BC, they split their forces into three divisions. One division was led by Cerethrius against the Thracians and Triballi; another by Bolgios against the Macedonians and Illyrians; and the third against Paionia by Brennus and Acichorius. Bolgios' expedition inflicted heavy losses on the Macedonians and killed their king, Ptolemy Keraunos, but was repulsed by the Macedonian nobleman Sosthenes. Brennus' contingent then attacked Sosthenes as defeated him, and proceeded to ravage the country. After these expeditions returned, Brennus urged a united, and potentially lucrative, attack on Greece, led by himself and Acichorius. The army numbered 152,000 infantry and 24,400 cavalry, Pausanias describes how they used a tactic called trimarcisia, where each cavalryman was supported by two mounted servants, who could supply him with a spare horse if he was dismounted, or take his place in the battle if he was killed or wounded, so the actual number of horsemen was in fact 61,200.[2]
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Bolgios (Greek Βόλγιος, also Bolgius, Belgius) was a Gaulish leader who led an invasion of Macedon and Illyria in 279 BC, killing the Macedonian king Ptolemy Keraunos.

He was part of a force of Gauls settled in Pannonia who had advanced to Thrace under a leader called Cambaules. Deciding on fresh conquests, they sent separate forces to different regions: one led by Cerethrius against the Thracians and Triballi; another against Paionia led by Brennus and Acichorius; and a third against the Macedonians and Illyrians, led by Bolgios.

The Macedonian king, Ptolemy Keraunos, was unconcerned by the approaching army, and declined an offer of 20,000 soldiers from the Dardanians. Bolgios sent ambassadors to Ptolemy, demanding payment to call off the attack. Ptolemy refused, instead demanding that the Gauls give him hostages and hand over their arms. Battle followed a few days later, where the Macedonians were severely defeated. Ptolemy was thrown by the elephant he was riding. He was captured and beheaded, and his head was parades around the army on a spear.

The Gauls did not follow up their victory, and Sosthenes, a Macedonian nobleman, took command, assembled an army and forced them to withdraw, although he was defeated by Brennus' contingent soon afterwards. The combined Gaulish army under Brennus and Acichorius then mounted an invasion of Greece, defeating a combined Greek army at Thermopylae and advancing to Delphi, where they were routed.[1]

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