10.20.1 Anyone who so wishes can compare the number of those who mustered to meet King Xerxes at Thermopylae with those who now mustered to oppose the Gauls. To meet the Persians, there came Greek contingents of the following strength. Lacedaemonians with Leonidas not more than three hundred; Tegeans five hundred, and five hundred from Mantineia; from Orkhomenos in Arcadia a hundred and twenty; from the other cities in Arcadia one thousand; from Mycenae eighty; from Phleious two hundred, and from Corinth twice this number; of the Boeotians there mustered seven hundred from Thespiae and four hundred from Thebes. A thousand men of Phokis guarded the path on Mount Oitē, and the number of these should be added to the Greek total.

10.20.2 Herodotus^* does not give the number of the people of Lokris who live at the base of Mount Knemis, but he does say that each of their cities sent a contingent. It is possible, however, to make an estimate of these also that comes very near to the truth. For not more than nine thousand Athenians marched to Marathon, even if we include those who were too old for active service and slaves; so the number of the fighting men of Lokris who marched to Thermopylae cannot have exceeded six thousand. So the whole army would amount to eleven thousand two hundred. But it is well known that not even these remained all the time guarding the pass; for if we except the Lacedaemonians, Thespians, and Mycenaeans, the rest left the field before the conclusion of the fighting.

10.20.3 To meet the barbarians who came from the Okeanos, the following Greek forces came to Thermopylae. Of the Boeotians, ten thousand hoplites and five hundred cavalry, the Boeotarchs being Kephisodotos, Thearidas, Diogenes, and Lysander. From Phokis came five hundred cavalry with footmen three thousand in number. The generals of the men of Phokis were Kritoboulos and Antiokhos.

10.20.4 The men from Lokris stationed over against the island of Atalanta were under the command of Meidias; they numbered seven hundred, and no cavalry was with them. Of the Megarians came four hundred hoplites commanded by Hipponikos of Megara. The Aetolians sent a large contingent, including every class of fighting men; the number of cavalry is not given, but the light-armed were seven hundred and ninety, and their hoplites numbered more than seven thousand. Their leaders were Polyarkhos, Polyphron, and Lakrates.

10.20.5 The Athenian general was Kallippos, the son of Moirokles, as I have said in an earlier part of my work,^* and their forces consisted of all their seaworthy triremes, five hundred horse, and one thousand foot. Because of their ancient reputation, the Athenians held the chief command. The king of Macedonia sent five hundred mercenaries, and the king of Asia a like number; the leader of those sent by Antigonos was Aristodemos, a Macedonian, and Telesarkhos, one of the Syrians on the Orontes, commanded the forces that Antiokhos sent from Asia.

10.20.6 When the Greeks assembled at Thermopylae^* learned that the army of the Gauls was already in the neighborhood of Magnesia and Phthiōtis, they resolved to detach the cavalry and a thousand light armed troops and to send them to the Sperkheios, so that even the crossing of the river could not be achieved by the barbarians without a struggle and risks. On their arrival, these forces broke down the bridges and by themselves encamped along the bank. But Brennus himself was not utterly stupid nor inexperienced for a barbarian in devising tricks of strategy.

10.20.7 So on that very night, he dispatched some troops to the Sperkheios, not to the places where the old bridges had stood, but lower down, where the Greeks would not notice the crossing, and just where the river spread over the plain and made a marsh and lake instead of a narrow, violent stream. Here Brennus sent some ten thousand Gauls, picking out the swimmers and the tallest men; and the Celts as a people are far taller than any other .

10.20.8 So these crossed in the night, swimming over the river where it expands into a lake; each man used his shield, his national buckler, as a raft, and the tallest of them were able to cross the water by wading. The Greeks on the Sperkheios, as soon as they learned that a detachment of the barbarians had crossed by the marsh, forthwith retreated to the main army. Brennus ordered the dwellers around the Malian gulf to build bridges across the Sperkheios, and they proceeded to accomplish their task with a will, for they were frightened of Brennus, and anxious for the barbarians to go away out of their country instead of staying to devastate it further.

10.20.9 Brennus brought his army across over the bridges and proceeded to Herakleia. The Gauls plundered the country and massacred those whom they caught in the fields but did not capture the city. For a year previous to this, the Aetolians had forced Herakleia to join the Aetolian League; so now, they defended a city which they considered to belong to them just as much as to the people of Herakleia.