Titus Livius (Livy), The History of Rome, Book 38
Evan T. Sage, Ph.D., Ed.

25. Ambassadors from the Tectosages came to the consul at his base at Ancyra, requesting that he should not move from Ancyra until he had conferred with their chiefs: there were no terms of peace which would not be preferable in their sight to war. [2] The time fixed was for the next day and the place one which seemed approximately half-way between the camp of the Gauls and Ancyra. [3] When the consul had come there at the designated time, attended by a guard of five hundred cavalry, and had returned to his camp without having seen any Gaul there, the same ambassadors returned, apologizing that their chiefs could not come by reason of religious objection; [4] the leading men of the tribe, they said, through whom the business could equally well be transacted, would come. The consul said that he too would send a representative, namely, Attalus. [5] Both parties attended this conference. When [p. 87]Attalus had brought up with him a bodyguard of¹ three hundred cavalry, terms of peace were discussed; [6] since a conclusion to the matter could not be reached in the absence of the principals, it was agreed that the consul and the chiefs should meet in that place the following day. [7] The evasiveness of the Gauls had this purpose: first, that they might waste time until they could transport their property, which they did not wish to jeopardize, together with their wives and children, across the river Halys; second, that they were plotting against the consul himself, who had not been sufficiently on guard against treachery at the conference. [8] For this purpose they chose from their entire number a thousand cavalry of tried boldness; and success would have attended their treachery had not fortune stood on the side of the law of nations which they had planned to violate. [9] The Roman foragers and wood-gatherers were led in the direction in which the conference was to be held, the tribunes thinking that this would be safer, since they would have between them and the enemy the consul's bodyguard, placed as a sort of outpost for them; nevertheless, they posted another outguard of their own, consisting of six hundred cavalry, nearer the camp. [10] The consul, on the assurance of Attalus that the chiefs would come and that the business could [11??] be settled, set out from camp with the same guard of cavalry as before, and, when he had gone about five miles and was not far from the appointed place, suddenly he saw the Gauls coming, their horses at full gallop, and with the air of enemies. [12] He halted his column, and, ordering the troopers to prepare arms and, minds for the combat, at first he stoutly received the onset of the attack and did not [p. 89]give way; then, when the weight of numbers was² bearing him down, he began to retire gradually, preserving the formation of his troops; finally, when there was now more danger in delay than protection in maintaining the formation, all scattered in random flight. [13] Then indeed the Gauls began to pursue and kill the scattered cavalry; and a great part of them would have been destroyed had not the outpost of the foragers, the six hundred cavalry, come up to them. [14] When they had heard from afar the terrified shouts of their comrades and had made ready their weapons and horses, they came upon the rout with their force still fresh. So straightway fortune changed and the panic changed sides, from the vanquished to the victors. [15] And at the first attack the Gauls were routed and the foragers flocked in from the fields and foes faced the Gauls from every side, so that they did not find even flight either easy or safe, since the Romans with fresh horses were pursuing the weary. And so few then escaped; no one was taken prisoner; by far the greatest part paid with their lives the penalty for breaking the faith of a conference. [16] The Romans, their hearts on fire with wrath, advanced the next day with their whole strength against the enemy.