The Celtic Invasion of Greece & The Unknown Battle of Thermopylae

200 years after the Persian Wars, Greece faced another massive assault. This time, the threat came not from the east but from a Celtic invasion.

Jul 2, 2024By Neil Middleton, MA Ancient History, BA History & Archaeology

celtic invasion greece thermopylae


The Celtic invasion of Greece and Macedonia in 280-79 BCE is less famous than the Persian Wars. This assault from the West in many ways echoed that from the East. The Greeks themselves certainly saw the similarities between the two and celebrated their victories over the East and West as the triumph of civilization over barbarians. At the beginning of the Hellenistic Age, the Celts swept down into the Hellenic world. The collision of these two worlds would have profound effects on both.


Celts, Gauls, and Galatians  

Celtic Warriors, by Angus McBride, Source: Arthive


The events of 280-79 BCE go by a variety of names. The invasion can be described as coming from the Celts, the Gauls, or the Galatians. These different terms are used to describe the same people who operated across a large span of time and terrain.


What we now refer to as the “La Tène culture” consisted of a group of similar communities spread across central Europe, from modern France to the Danube and beyond. Covering the late Iron Age from roughly the 5th-1st centuries BCE, the term La Tène culture is an archaeological designation based on a similar material culture found across a wide area. Given the absence of written records, our sources are either archaeological or else come from the accounts of the Greeks and Romans.


Those Greeks and Romans ultimately defeated the Celts and left us with the image of a typical “barbarian.” To their enemies, the Celts were not without nobility and virtue but they were generally wild, ill-disciplined, fickle, and almost childishly simple people. While trade and contact were common over the centuries, the main meeting place between the Celtic and Mediterranean worlds that survives in our histories was the battlefield. The Romans never forgot the sack of Rome by the Gauls under Brennos in 390 BCE. Having inflicted damage and defeats on the Greeks and the Romans it is not surprising that the Celts gained such a negative image but we should bear in mind that Celtic society grew and evolved in parallel with those around it and its people were often capable of coming out on top.

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Celtic expansion during the Iron Age, Source: Wikimedia Commons


At the same time that the Greeks were colonizing the Mediterranean and the Romans were spreading across Italy, the Celts were migrating across Europe. From their heartlands around Switzerland, Germany, and eastern France, the Celts raided and migrated, as groups merged, split, and conquered over a vast area. This created a dispersed network of connected groups rather than one united political entity. Having established new communities in the northern Balkans and around the modern areas of Hungary and Serbia in the 4th century, there was no obvious reason why the Celts would not continue their movements further south.


The Celtic groups in the Balkans do not seem to have shown any interest in challenging the strong, world-conquering Macedonia of Alexander. The first contact between Celts and Macedonians was likely a diplomatic exchange in 335 BCE during Alexander the Great’s campaign in the Balkans. This first contact was followed by a generation of peace. Once the situation changed, however, with the turn of the 3rd century, new opportunities suddenly opened up.


The Invasion of Macedonia

The Battersea Shield, La Tène object found in London, 350-50 BCE, Source: The British Museum


The Celts kept their distance from Alexander’s Macedonia but the decades of war that followed his death left the country vulnerable. Several kings and contenders had come and gone over the years and many of the country’s soldiers were drawn away to new opportunities or had died on battlefields across Asia. In 280 BCE, the new King of Macedonia was an exiled opportunist.


Ptolemy Keraunos was an exiled former heir to the throne of the Ptolemaic Empire in Egypt. He owed the nickname Keraunos (Thunderbolt) to his recklessness and impatience. He seized the throne by treacherously murdering Seleucus I who was then on the verge of taking control of Macedonia. More deaths followed, and, as Keraunos consolidated his hold but barely a year into his reign, a new threat emerged.


With its armies depleted, frequent civil wars, and a rash unpopular king, Macedonia must have looked vulnerable in 280 BCE. For the Celts, who had already started looking south, the moment was not to be missed. According to our main source for these years, the later writer Pausanias (10.19), the Celts were motivated by the desire for plunder. There is no reason to doubt this motive but the Celts may well have had other goals in mind as a combination of raiding and settling had brought them from Western Europe to the Balkans.


Tempting as Macedonia was, it was not the only target. Three attacks were planned across the Balkans with one force heading east against the Thracians, and another aimed at the Paionians, while a third force under the leadership of Bolgios struck Macedonia.


Gold Stater of Ptolemy Keraunos, Source: Classical Numismatic Group


Not much is known about the other sections of the Celtic force but those soldiers under Bolgios reached Macedonia and confronted Keraunos’ army. Bolgios offered peace in exchange for being paid off. Keraunos refused and met the Celts in battle (Justin, 24.4-5).


The outcome was a disaster, with the Macedonians defeated and Keraunos himself killed and beheaded. The Celts did not push their advantage, however. Pausanias states that they lacked the numbers to continue to plunder Macedonia and Bolgios’ army would have struggled to take the cities the remaining Macedonian soldiers retreated into (Pausanias, 10.19.4). The invasion of Macedonia may have been limited but the defeat of Keraunos removed an obstacle. There was now little standing between the Celts and the rich cities of Greece.


The Battle of Thermopylae

The Ludovisi Gaul, 2nd century CE, Source: Arthive


Having swept aside the Macedonians the Celts were now a major threat to Greece. Bolgios had not capitalized on his victory but others would not let the moment slip. The Celts in the Balkans seem to have been led by a warrior aristocracy that could gather an army around themselves and they did not have a monarch. One of these leaders was a particularly charismatic man the Greeks knew as Brennos.


In the councils of the Celts, Brennos argued for an invasion of Greece and successfully brought together a huge army. Numbers in ancient literature are unreliable and Pausanias’ figure of 152,000 infantry and 20,400 cavalry can be doubted (Pausanias, 10.19.6). Even if overestimated, this army outnumbered the Greeks and likely drew on the wider Celtic networks stretching into Germany and Italy. Later stories would link the invaders of Greece with Gallic groups in southern France. Other communities in the Balkans would also have joined, willingly or not, the advancing Celtic band.


Gaulish mercenary, from Egypt, 220-180 BCE, Source: The British Museum


The multitude of Greek states south of Macedonia now faced their greatest external challenge since the great Persian invasions two hundred years before. This was a Greece that had been struggling with mixed results for half a century to free itself from Macedonian control. Just like Keraunos’ kingdom, the Greek states were depleted by these long wars and were more divided than ever. The obvious danger though left the Greeks with no choice but to mount a defence.


There was a natural rallying point for those Greeks willing to resist the Celtic advance: Thermopylae. The narrow road between the mountains and the sea at Thermopylae had been made famous by the Spartan last stand against the Persians in 480 BCE. The pass forced any army invading from the north through a narrow gap only 300 feet wide. Thermopylae was by no means impossible to take and there were ways around the pass but all large armies had to get through here somehow and the advantage would always be with the defender.


In 279 BCE, those defenders were a coalition of central Greek states. To hold back Brennos’ army of over 150,000 the Greeks mustered between 25-30,000 with the main contingents coming from the central Greek states immediately in the firing line, the Aitolians, Boiotians, and Phokians. It has been suggested that Pausanias went too far in imitating the narrative of the Persian Wars by giving a prominent role to the Athenians. While certainly joining the defense, the Athenians of 279 BCE were a shadow of their former power and the leadership of the coalition was most likely with the Aitolians who held much of central Greece.


A view of Thermopylae, Source:


Brennos reached Thermopylae with little difficulty, as many of the surrounding Greek communities had little choice but to surrender. The battle of Thermopylae began early in the morning as the Celts massed and charged at the Greek heavy infantry. The Celts preferred tactic involved a mass charge of sword-wielding warriors. In the open spaces of Macedonia, this had been enough to overwhelm Keraunos’ army, but in the narrow confines of Thermopylae the Greek hoplites held their ground while light infantry showered down missiles. After several charges, the Celts fell back with heavy losses while only 40 Greeks were killed.


If a frontal assault would not work Brennos had another idea. With the key contingent in the Greek army being the Aitolians, Brennos sent around 40,000 of his men to attack Aitolia directly. The Celts moved quickly and brutally, sacking the Aitolian city of Kallion and massacring men, women, and children. This display of ferocity was effective and the Aitolians left Thermopylae to defend their homes.


In this desperate moment, the Aitolian population mobilized en masse with men and women taking to the mountains to fight the Celts. The Aitolians had long been adept at warfare in their native mountains and the invading force quickly began to suffer. Attacking from the high ground the women and men of Aitolia destroyed a large part of the force sent out by Brennos.


Despite the heavy losses Brennos’ move had worked. With the Aitolian departure reducing the number of defenders at Thermopylae, Brennos found the paths through the mountains and got around the Greek defenses. Before they were surrounded the remaining Greek coalition soldiers withdrew. The Celts had breached Thermopylae and Brennos knew exactly where he wanted to target next.


The Assault on Delphi  

Temple of Apollo, Delphi, author’s photo


The obvious place to head to when looking for portable wealth in Greece was the sanctuary of Apollo at Delphi. One of the most renowned religious sites in the whole Mediterranean, it had for centuries housed precious dedications from the Greek world’s wealthiest. How much of this wealth was still present in the 3rd century is debatable but for Brennos it was an opportunity not to be missed.


After dislodging the Greeks from Thermopylae, Brennos took a part of his forces and headed straight to Delphi. The Aitolians were mostly occupied with skirmishing, with the remaining Celtic groups leaving the defence of Delphi in the hands of a smaller Aitiolian contingent, the Delphians and the Phokians. The Greeks would later recount the battle around Delphi as a miraculous event in which gods and heroes joined the ranks of the defenders. Disentangling the mythology from the reality of events is not possible at this distance.


Reconstruction of the sanctuary of Apollo at Delphi, by Albert Tournaire, 1894, Source: Wikimedia Commons


In Pausanias’ account the gods made their presence felt as Brennos climbed Mt Parnassus (Pausanias, 10.23.3). First there were earthquakes. Storms and lightning followed, greatly confusing the Celts. There may then have been an attempt to storm Delphi which failed as the priests and priestesses encouraged the defenders and claimed to see apparitions of gods and heroes joining the battle (Justin, 24.8). So mixed with myth did the story become that it is not clear whether the Celts managed to plunder the sanctuary or not. Frost, snow, and rockfalls now made themselves felt as the elements turned on the Celts.


At dawn, the Greeks used their knowledge of the terrain to attack the Celts from the mountain heights, raining down arrows and javelins. The Celts suffered heavily and would have struggled to come to grips with their assailants. Brennos himself was wounded a number of times. Demoralised and perhaps in retreat, a night of chaos followed the day of battle. As they camped for the night some kind of tumult broke up the Celtic force. The Greeks explained this as a divinely inspired panic or the effect of large amounts of wine the gods had cunningly placed in the Celts’ path (Justin, 24.7).


The Dying Gaul, Roman marble copy of Hellenistic original, Capitoline Museum Rome, author’s photo


The Celtic army was badly battered in the mountains around Delphi and either split into smaller groups to retreat or become disunited. Brennos had survived the battle but, badly wounded, urged his followers to leave behind the dying and flee while he killed himself. The Greeks continuously harassed the weakened and divided Celtic force. As news of the victory spread, more Greek forces arrived to finish off the Celts from Athens and Boeotia. The remaining Celtic bands seem to have been isolated and starved. Pausanias (10.23.6) estimated that more than 20,000 Celts had died in the attack on Delphi and the subsequent retreat. The remaining Celts fell back north, harassed all the way by the Greeks.


Aftermath: Glory and Galatians

Pergamon Altar, 2nd century BCE monument commemorating Pergamon’s victories, Pergamon Museum Berlin, Author’s photo


The Celtic invasion of Greece was over, but the Celtic presence would persist. Having failed to plunder Greece, future movements were directed eastwards. Within a few years the Celts had crossed the Hellespont into Asia and went on to found a new state in the heart of Anatolia: Galatia. The Galatians continued to speak their Celtic language in their new home in Asia for centuries. Celtic mercenaries became a common part of many Hellenistic armies. The Galatians remained a presence in the region when it came under Roman control in the 1st century BCE.


The experience of 280-79 BCE had a profound impact on the Greeks and Macedonians. In Macedonia a new king, Antigonus Gonatas, would finally emerge and found a dynasty in part due to a victory over a small Celtic group. Having played a leading role in the victory, the Aitolians became the principal power in central Greece for the next century.


With the Celtic invasion being seen as a replay of the Persian Wars, there was a rush to share in the glory. Each community that won a battle against a Celtic band turned the event into a focal point for their propaganda, making the figure of the Celt a central part of Hellenistic art. Though the invasions of 280-79 BCE were brief affairs, they led to an intertwining of the Celtic and Hellenic worlds which lasted for centuries.

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By Neil MiddletonMA Ancient History, BA History & ArchaeologyNeil Middleton has studied ancient history and archaeology up to Masters level (MA in Ancient History from the University of Wales Trinity Saint David) with a focus on ancient Greece. His particular areas of interest are the politics of the Greek world in the Classical and Hellenistic era. After his studies he has spent time living in Greece and France.