According to the narrative of Pausanias, who introduces the story as a digression in his description of Delphi, the Gauls invaded Greece under the leadership of a certain Brennus, the same name, it will be observed, as that borne by the conqueror of Rome (the word Brennus has been said to mean "king"; but Celtic scholars are not agreed upon the point). His forces are said to have amounted to 150,000 infantry, a figure on which the authorities are fairly unanimous, and cavalry variously estimated at from 60,000 to 10,000. The Greeks, though in a very depressed condition, roused themselves to resist. It was not a choice, as it had been two centuries before, between freedom and servitude; it was a question of life or death. The barbarians spared no one, and if they could not be checked in their advance, Greece would be turned into a desert. The stand was to be made, as of old, at Thermopylę. The comparison between the forces led by Leonidas and those now assembled is interesting. The most numerous contingent was from a nation which scarcely appears in the history of Greece at its best days, the Ętolians. "Very numerous and including every arm," says Pausanias. Their heavy-armed infantry numbered 9,000. The other figures he does not give, or they have disappeared from his text. The whole force may have amounted to between thirty and forty thousand.

A battle that was fought in the Pass ended greatly to the advantage of the Greeks. The Gauls with their long and unwieldy swords and cumbrous shields were no match for their antagonists, though they fought with desperate valour. Their cavalry, the strongest arm they possessed, could not act on account of the nature of the ground. The result was that they were driven back with very heavy loss, while the Greeks had but forty killed.

Brennus, who seems to have had some military ability, seems to have become aware that the Ętolians made up the most numerous and effective part of the Greek army. He conceived the idea of detaching them by sending a force under his second-in-command to ravage Ętolia. The stratagem succeeded. The Ętolians, on hearing of the movement, hastened to march to the defence of their country. They were too late to save two of their frontier towns, which were stormed and sacked in the most brutal manner. But they were in time to exact a heavy vengeance from the barbarians. Of the fifty thousand who had been detached on this expedition, less than half returned to the camp at Thermopylę.

The incidents that followed bear a curious resemblance to the history of the first defence of Thermopylę. The path by which the Persians, through the treachery of Ephialtes, were able to take the defenders of the pass in the rear was again used for the same purpose. The Phocian pickets were surprised as before, being hindered by the mist from seeing the Gauls till these were close upon them. But there was no obstinate determination among the Greeks to die upon the ground. They were carried off by the Athenian fleet, which from the first had been in attendance, keeping as close as possible to the shore.

The object which now roused the cupidity of the barbarians was the shrine of Delphi with its treasury, still rich in the offerings of many generations of worshippers and inquirers, though it had not altogether escaped the hand of the spoiler.

As in the Persian war, the terrified inhabitants inquired of the god whether they should remove or conceal the sacred treasure. Again, as before, the answer was that the god would take care of his own. "I will provide, and with me the Maidens veiled in white," were the words of the oracle. The greater part of the army mustered at Thermopylę had gone home; but there were some thousands who remained to protect Delphi. The god did not disdain to use their services, though the most effective protection came—so runs the story—from his own interference. The ground on which the Gauls had pitched their camp was shaken throughout the day by repeated shocks of earthquake, while overhead the thunder rolled and the lightning flashed incessantly. Through the darkened atmosphere might be seen the flashing arms of warriors who were more than mortal—one of them, it was said, the hero Pyrrhus, son of Achilles, who had met his death at Delphi many centuries before, and had ever since been worshipped as a local hero.

That day, however, the Gauls held their own; many of the Phocians, in particular, were slain. But the night that followed was one of terrible suffering. A sharp frost set in, and following the frost came a heavy fall of snow. The snow symbolised "the maidens vested in white"—such, at least, was the rationalistic explanation given in after years. Nor was this all: great masses of stone from Parnassus, and rolling into the camp of the barbarians crushed as many as twenty or thirty by a single blow. The next day the Greek garrison at Delphi advanced against the invaders, the main body making a front attack, the Phocians, who were well acquainted with the country, assailing the rear. The Gauls did not lack in courage or firmness. Suffering though they did intensely from the cold, they made a resolute stand, and did not retreat till their leader was severely wounded and carried fainting off the field. Again the night was more fatal than the day. After dark a panic fear fell upon the camp. The barbarians seemed to see and hear enemies everywhere, and turned their arms upon each other. After this their destruction was certain. To a host without discipline a retreat is fatal. The Gauls were without stores, for they reckoned to be supported by the countries through which they passed. But now the victorious enemy hung upon their rear, and cut off any stragglers that ventured to leave the main army. Famine and the incessant attacks of the pursuers reduced their numbers till there was but a scanty remnant of the great host that a few weeks before had descended on Northern Greece. Brennus, it is said, poisoned himself, unable to face his people at home after so disastrous a campaign.

Pausanias tells us that not one of the invading Gauls quitted Greece alive. It is hardly probable that this is true; and other writers gave a different account. What is certain is that one great division of the swarm that had descended from Northern into Southern Europe met with a very different fortune from that which overtook Brennus. This took a more easterly route, and plundering and destroying as it went reached the shores of the Hellespont. (This seems to have happened in 278 , the year after that in which Delphi had been attacked.) The Gauls cast covetous glances on the rich territories of Asia, now separated from them by only a narrow stretch of water, and in one or another contrived to reach them. One division seized a few small vessels and boats, and, as no sort of opposition was attempted, ferried themselves across; the other was actually transported by an Asiatic Greek prince, who was contending with his brother for the kingdom of Bithynia. They secured the victory for him, but Bithynia, and indeed the whole of Western Asia Minor, paid a heavy price for their help. Their history during the next few years is very obscure, but we may gather that they roamed from province to province, laying waste all the countries which they traversed. The unwarlike inhabitants of Asia Minor were quite powerless to check them. After some twelve years Antiochus, King of Syria, son of one of the great generals trained by Alexander, undertook the task, and accomplished it with such success that he earned the surname of Soler, "the Saviour." He could not indeed expel them; in fact, so far was their power from being broken that in 261 Antiochus lost his life in a battle with them. But the general result of the war was that the invaders were glad to settle down in a definite region which was ceded to them, and which was known by the name of Galatia, or Gallo-pęcia. The Galatians afterwards played an important part in history. But with this we are not now concerned.