Just when the situation in the hellenistic empires seemed to be stabilized, the Galatians invaded Greece. The deepest cause was the collapse of the kingdom that had once been Lysimachus': when this king was killed in 281, the tribes in the buffer zone in the north sided with the barbarous Galatians, who now found the way to the south open. These Galatians belonged to the La Tène-culture, which is often called 'Celtic'.

The story is told by Pausanias (Guide for Greece 10.19.4-23.9); his source may or may not be Hieronymus of Cardia. The translation was made by Peter Levi.

 	
"I wanted to bring out the story of the Celtic invasion more clearly in my account of Delphi, because this was where the Greeks did most against them.

"The Celts made their first expedition under the command of Cambaules; they got as far as Thrace, but despaired of the way ahead as they realized there were only a few of them and they were no match for the Greeks in terms of numbers.[1]

"When they decided a second time to carry arms against foreign countries (driven most of all by veterans of Cambaules' campaign who had tasted piracy and fallen in love with the loot and rape of the world), they came in a mass of infantry and a very considerable throng of cavalry as well. The commanders divided the army into three parts, each to advance into a different country.

"Cerethrius was to lead against the Thracians and Triballians [2],
while Brennus [3] and Acichorius commanded the advance into Paeonia; Bolgius marched against the Macedonians and Illyrians and undertook a struggle with Ptolemy, who at that time was king of Macedonia. (This was the Ptolemy who treacherously murdered Seleucus son of Antiochus after taking refuge under his protection, and was called the 'Thunderbolt' for his utter daring.) Ptolemy himself died in the battle [4] and there was a sizable massacre of Macedonians, but even then the Celts did not have the confidence to advance against Greece; and so the second expedition returned home.

"But Brennus was a powerful influence on their general councils and individually on all the men in authority, calling for a campaign against Greece, and pointing out the weakness of the Greeks at that time, their great public wealth, and the even greater wealth in the sanctuaries in dedications and in coined silver and gold. He persuaded the Gauls to march on Greece, and among the fellow-commanders he chose from the great men of Gaul was Acichorius. 

"The army that gathered was a 152,000 infantrymen and 20,400 horsemen: that was the number of the cavalry always in action, but the real number was 61,200, as there were two grooms to each horseman, all mounted and good riders. When the Gaulish cavalry were in battle, the grooms would stay behind the ranks and make themselves useful with new mounts when a horse or rider fell, but when a man was killed the slave would mount the horse in place of his master. If man and horse both died he was ready mounted. When they were wounded, one of the slaves took away the wounded man to camp, while another stepped into the line in his place. [...] In their own language they called this division trimarkisia; you should realize marka is the Celtic word for 'horse'. This was the armament and this was the resolution with which Brennus marched on Greece.  	 
	
"Greek spirit had sunk right down, but the power of fear forced them to realize that Greece must fight. They saw that this struggle was not about freedom as it once was against Persia; it was not going to be enough now to offer earth and water [5]. What had happened to Macedonia, to Thrace, to Paeonia, in the previous onslaught of the Gauls, was still in their memory, and news came of. the outrages that were now being committed in Thessaly. Every man as an individual and every city collectively had realized that the Greeks must overcome or be destroyed. [...]

"Here is the number of Greeks who appeared at Thermopylae [6] against the barbarous tribes of the Atlantic ocean.

    * 10,000 men at arms and 500 cavalry from the Boeotians, under the captains of Boeotia: Cephisodotus, Thearidas, Diogenes, and Lysander;
    * 500 horsemen from Phocis and infantry amounting to 3000, under Critobulus and Antiochus;
    * 700 Locrians from the island of Atalante under Meidias, with no cavalry;
    * 400 regimental soldiers from Megara,
    * and an Aetolian contingent on the largest scale of any, with the number of cavalry unrecorded, 790 light infantry and more than 7000 regimental soldiers, under Polyarchus, Polyphron and Lacrates;
    * the Athenian commander was Callippus (as I explained earlier) with a force of all the serviceable warships, 500 cavalry and a 1000 in the infantry, and her ancient prestige gave Athens pride of place.
    * The kings [Antigonus Gonatas and Antiochus I Soter] sent mercenaries, 500 from Macedonia and as many again from Asia; Antigonus' officer was Aristodemus of Macedonia, and Antiochus' officer who also commanded the Asians and the Syrians from the Orontes was Telesarchus.

"When the Greeks had assembled at Thermopylae and discovered that the Gaulish army was already in Magnesia and Phthiotis, they decided to send out their cavalry with a thousand light infantry to the Spercheius [7], and not to let the barbarians cross the river in security without a fight. They broke down the bridges and encamped on the riverbank.

"But Brennus was not so barbarous as to be completely unsophisticated or wholly inexperienced in the contrivances of war. As soon as darkness fell, Brennus picked 10,000 Gauls who could swim or had more than average height (and to start with Celts are the tallest people in the world) and sent them not to the ancient crossing-places, but lower down where the Greeks would not know they were crossing, to where the Spercheius broadens out over level ground, forming a marshy lake instead of a narrow, violent current. So they got across under cover of darkness by swimming the marshy part of the river, using the oblong shields of their country like rafts, and the tallest ones simply wading across. When, the Greeks at the Spercheius discovered that a party of barbarians had crossed the marsh they retreated into the main camp.

"Brennus gave orders to the people of the Malian gulf to bridge the Spercheius, and they finished the job with an enthusiasm born of terror and of a longing to see the barbarians safely off their territory. Brennus took his army across the bridge and turned on Heraclia [8]; the Gauls plundered the countryside and murdered the people they caught out in the fields but failed to capture the city: the year before this the Aetolians had forced Heraclia into the Aetolian League, and now they defended it as a city that belonged as much to them as it did to its own people.

"But Brennus did not care so much bout Heraclia; he was struggling to drive the enemy that faced him out of the passes and to reach the rest of Greece beyond Thermopylae. He advanced from Heraclia, knowing from deserters what men from each city had gathered at the Gates [9], and in contempt of these Greeks he opened a battle as the sun rose the next morning; even supposing the Celts have some art of divination of their own, Brennus had no Greek soothsayer and made no concession to local religious observance. The Greeks advanced in silence and in order, and engaged hand to hand without the infantry breaking rank even enough to disturb their fighting formations, and with light infantry holding position, shooting, slinging, and throwing javelins. Cavalry was no use on either side because the Gates are a narrow pass, and the ground is broken and slippery, a continuous series of streams among rocky outcrops.

"The Gaulish equipment was weaker, as the traditional oblong shields they carried were all the protection their bodies had; in military experience they fell even further behind. They rushed at their adversaries like wild beasts, full of rage and temperament, with no kind of reasoning at all; they were chopped down with axes and swords but the blind fury never left them while there was breath in their bodies; even with arrows and javelins sticking through them they were carried on by sheer spirit while their life lasted. Some of them even pulled the spears they were hit by out of their wounds and threw them or stabbed with them.

"Meanwhile the Athenians on the warships with some difficulty and danger sailed in through the mud-banks that stretch far out to sea, and held their ships close inshore, bombarding the flanks with all kinds of arrows and weapons. The Celts were indescribably tired, and being in a narrow place they had little effect while they suffered two or three times as much, so their commanders gave them the signal to retreat to camp. As they turned away in broken ranks and no kind of order, many of them were trampled under each other's feet, and many went into the swamp and disappeared in the mud; as many of them perished in the retreat as died in the height of battle.

"On that day the Athenians showed the greatest courage in Greece, and the bravest of them was Cydias, a young man in battle for the first time. The Gauls killed him, and his kinsmen dedicated his shield to Zeus of Freedom, with this inscription.

    'The shield of a brave man, Zeus' offering,
    pining away for the youth of Cydias:
    the first shield his left arm ever put on,
    hen raging War went hottest at the Gauls.'

"This was the inscription in the days before Sulla's men took away the shields from the colonnade of Zeus of Freedom among the loot of Athens [10]. And now at Thermopylae after the battle the Greeks buried their dead and stripped the barbarians, but the Gauls sent no herald for the taking up of their dead, not caring whether they were buried or fed wild animals and the birds who make war on corpses. This neglect of giving graves to those who had passed away was for two reasons I think: to astound their enemies, and because they have no natural pity for the dead. Forty Greeks died in the battle, but it was impossible to discover the exact number of barbarians, since a large number of them vanished into the mud." 

 Note 1:
This happened in the spring of 279.

Note 2:
The Triballians were a tribe to the north of Macedonia.

Note 3:
It is possible that brennus means something like 'duke'.

Note 4:
In 279. He had murdered Seleucus in 281.

Note 5:
In the war against the Persians (which culminated in the years 480-479, when king Xerxes invaded Greece), offering earth and water had been sufficient as signs of submission. Xerxes did not loot his own subjects.

Note 6:
A narrow road between mountains and sea, where a small army could prevent a large army from entering Greece.

Note 7:
A river near Thermopylae.

Note 8:
The last town before Thermopylae. 

Note 9:
Thermopylae means 'hot gates'.

Note 10:
The Roman general Sulla captured Athens in 86 BCE. 