Titus Livius (Livy), The History of Rome, Book 5
Rev. Canon Roberts, Ed.

38. The consular tribunes had secured no position for their camp, had constructed no entrenchments behind which to retire, and had shown as much disregard of the gods as of the enemy, for they formed their order of battle without having obtained favourable auspices. [2] They extended their line on either wing to prevent their being outflanked, but even so they could not make their front equal to the enemy's, whilst by thus thinning their line they weakened the centre so that it could hardly keep in touch. On their right was a small eminence which they decided to hold with reserves, and this disposition, though it was the beginning of the panic and flight, proved to be the only means of safety to the fugitives. [3] For Bennus, the Gaulish chieftain, fearing some ruse in the scanty numbers of the enemy, and thinking that the rising ground was occupied in order that the reserves might attack the flank and rear of the Gauls while their front was engaged with the legions, [4??] directed his attack upon the reserves, feeling quite certain that if he drove them from their position, his overwhelming numbers would give him an easy victory on the level ground. So not only Fortune but tactics also were on the side of the barbarians. [5] In the other army there was nothing to remind one of Romans either amongst the generals or the private soldiers. They were terrified, and all they thought about was flight, and so utterly had they lost their heads that a far greater number fled to Veii, a hostile city, though the Tiber lay in their way, than by the direct road to Rome, to their wives and children.

For a short time the reserves were protected by their position. [6] In the rest of the army, no sooner was the battle-shout heard on their flank by those nearest to the reserves, and then by those at the other end of the line heard in their rear, than they fled, whole and unhurt, almost before they had seen their untried foe, without any attempt to fight or even to give back the battle-shout. [7] None were slain while actually fighting; they were cut down from behind whilst hindering one another's flight in a confused, struggling mass. [8] Along the bank of the Tiber, whither the whole of the left wing had fled, after throwing away their arms, there was great slaughter. Many who were unable to swim or were hampered by the weight of their cuirasses and other armour were sucked down by the current. [9] The greater number, however, reached Veii in safety, yet not only were no troops sent from there to defend the City, but not even was a messenger despatched to report the defeat to Rome. [10] All the men on the right wing, which had been stationed some distance from the river, and nearer to the foot of the hill, made for Rome and took refuge in the Citadel without even closing the City gates.