The Celtic Invasion of Greece | History Hit

The Celtic Invasion of Greece

Image Credit: Public Domain

In 279 BC the once-dominant kingdom of Macedonia lay in ruins, ravaged by a massive Celtic army. Celtic tribes had ventured into southern Europe during the 5th and 4th centuries BC. After the death of Alexander the Great in 323 BC and the Successor Wars that followed, Macedonia and the Greek city-states beyond seemed like rich pickings.

Brennus, the leader of the Celtic army, was not satisfied with simply raiding Macedonia. Eager for fame and glory, he marched south towards Greece’s most famous city-states – poleis such as Athens, Sparta, Corinth and Thebes. These cities had once epitomised the zenith of power in the Hellenic World. But by 279 BC, times had changed.

Ever since King Philip II‘s victory at Chaeronea in 338 BC, many of these city-states had been subject to Macedonian control. Following Alexander the Great’s death in 323 BC, many found themselves dragged into brutal conflicts between various generals in the Successor Wars. This had taken a toll on their military power.


The fate of Macedonia

News reached the city-states of events in the north. Macedonia was in turmoil – the country pillaged, its people in despair and its army vanquished. More disturbing was that Brennus and his Celtic horde were marching south, leaving a trail of death and destruction in their wake. Greece was in jeopardy.

Once more, the famed Greek cities of old were under threat from a barbarian horde and the need for Hellenic unity became apparent. Yet creating a panhellenic defence was no easy matter. Rivalry was common between many of these cities by 279 BC. For some, the thought of serving alongside forces from another city would have been almost unbearable – how could they serve alongside men who had been their most bitter enemies?


But with the impending arrival of Brennus’ marauding horde, differences were temporarily cast aside and war preparations were made. Their aim was simple: defend the heartlands of Hellenism from the barbarians and survive. Spearheading this effort were the Athenians.

The Athenians knew that no matter how many Greeks answered the call, the great horde would still dwarf their force in size. They thus sought a battlefield to counter this disadvantage – a place where numbers would count for little. One place suited that style of warfare more than any other.


Famed as the spot where 300 Spartans held off a Persian army, the pass at Thermopylae had come to epitomise Greek defiance against overwhelming odds. Its narrow passage, blocked on one side by the mountains and the other by the sea, provided a defending army with two significant advantages. Not only did its limited space mean that they required only a small force to defend the pass, but it also rendered cavalry warfare almost useless: horsemen could neither efficiently charge nor flank their foe in such a restricted area.

1876 depiction of Thermopylae’s coastline in the time of Herodotus compared with the contemporary coastline.

Image Credit: Public Domain

For Callippus, the Athenian commander, this was ideal. Although his force did consist of a small contingent of formidable cavalry, most of the Greek army consisted of hoplites, heavily-armed infantrymen equipped with spear and shield. These men were ideal for close-combat fighting in defensive phalanx formations. Holding ground was their speciality.

The perfect defence

As for Brennus, Callippus knew fighting at Thermopylae would give his foe great difficulty. Its narrow passage meant the formidable size of his force would become more of a hindrance than a benefit, their weight of numbers forcing their front ranks into the deadly doru-points of the hoplite phalanx. Meanwhile the famed Celtic cavalry – some of the most formidable horsemen in antiquity – would be rendered next to useless.

The Athenians quickly reached a decision. Fighting at Thermopylae, they knew, was the best chance a united Greek force had at defeating the impending horde; here they might just pull off a miracle. 

The army gathers

Hellenic forces from across the Aegean answered the summons. Boeotians, Aetolians, Locrians, Phocians and Megarans – all arrived at the pass with troops to aid the war-effort. That was not all.

Ever hoping to improve their standing with the Greek cities, support from powerful Hellenistic warlords would also arrive to face the impending threat: from King Antiochus I, ruler of the supreme Seleucid Empire in Asia and from Antigonus Gonatas, the son of the famed Demetrius Poliorcetes. They had answered the call.

2,499 years ago one of the most famous battles in antiquity, indeed one of the most famous battles in history, was fought at the Pass of Thermopylae in central Greece. King Leonidas, his 300 (or so) Spartans and their Hellenic allies fought off against King Xerxes' mighty Persian army for three days. We're joined by Paul Cartledge, a professor from the University of Cambridge and one of the world's leading experts on ancient Sparta, to sort the fact from the fiction about this iconic clash.
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The defending force at Thermopylae soon swelled. In total, Callippus had over 25,000 infantry, 2,000 cavalry and a handful of ships under his command – no insignificant sum. Yet would it be enough to overcome the impending threat? Only time would tell.

Alarming news soon reached the Greek defence: Brennus and his Celtic horde were near. Already Thessaly had succumbed to the barbarian menace without a fight, its lands plundered and people in despair. And now, with the route to Boeotia more open than ever, Brennus was barely days away from reaching the Malian Gulf and Thermopylae. Time was running out.

Preparing the defences

Callippus and the Greeks knew how the last Greek defence at Thermopylae had been overcome. The famed mountain path across Mt Oeta – which had been used by Hydarnes and his Persian force all those years before – was common knowledge to them. If unguarded, this path, although difficult to traverse, could allow an army to both bypass and surround the defending force in the Hot Gates. The result would be Greek slaughter.

Yet for the moment, Brennus and his approaching Celts were unaware of this one fatal flaw. But Callippus would take no chances. He ordered two of the Greek forces to prevent Brennus bypassing the main defence. The Greek flanks were now secure, with Brennus’ arrival imminent. Yet the Greeks had one more card to play to complete their preparation.

The Sperchius

Hoping to hinder the enemy force before their arrival at the Pass, they dispatched their horsemen and lightly-armoured soldiers further north, to the southern bank of the Sperchius River. Their task was to guard the Sperchius, a river the Celtic horde had to pass over if they were to reach Thermopylae. 

Brennus and his horde soon arrived at the River’s northern bank to an unwelcome site. Their path ahead was blocked: all the bridges had been destroyed, while lightly-armed Greeks were visible on the far-bank – taunting and ready to rain death on any Celtic attempts to cross the river.

Brennus knew that overcoming this barrier would be difficult. Any attempt to ford the river in daylight would inevitably prove very costly for his men: they would be sitting ducks in the water – target practice for the Greeks opposing them. This was what the Greeks had hoped Brennus would do. Yet Brennus had other ideas.

After Brennus and his 10,000 Celts crossed the Sperchius, the Greek vanguard retreated to Thermopylae.

The night crossing

Waiting until darkness, Brennus selected 10, 000 of his soldiers – chosen for their height and swimming ability – and ordered them to an unknown crossing point near the mouth of the Sperchius. The Greeks, believing no such crossing existed, remained completely unaware.

Using their iconic oblong shields to help them, thousands of Celts floated silently across the Sperchius – no small military feat. The first layer of the Greek defence had been breached. Roused from slumber to learn 10, 000 angry Celts were bearing down on them, the Greek vanguard hastily retreated to Thermopylae. It was either that or suffer a gruesome end. Taking full advantage, Brennus ordered the bridges to be rebuilt and duly crossed over with his army. The road to Thermopylae now lay open. 

Tristan Hughes