Titus Livius (Livy), The History of Rome, Book 23
Frank Gardener Moore, Professor Emeritus in Columbia University, Ed.

29. They had their camps five miles apart for a few days, not without skirmishes, but without drawing up lines of battle. [2] Finally on one and the same day, as though by agreement, the signal for battle was raised on both sides and with all their forces they went down into the plain. The Roman line stood in triple ranks. [3] Some of the light-armed were posted in the intervals between the maniples in advance of the standards, some placed behind the standards. [4] Cavalry covered the wings. Hasdrubal made a strong centre of Spanish troops; on the right wing he placed Carthaginians, on the left Africans and mercenary auxiliaries. Of the cavalry he stationed the Numidians on the wing of the Carthaginian infantry, the rest on that of the Africans. [5] And not all of his Numidians were placed on the right wing, but only those who, taking two horses apiece¹ after the manner of performers, had the custom of leaping armed from the tired horse to the fresh, often in the very heat of battle; such was the agility of the men, and so well-trained their breed of horses. [6] While they were standing in this array, the hopes of the generals on the two sides were fairly balanced; for there was also not much superiority for the one army or the other either in the number or the type of its soldiers. [7] But the spirit of the soldiers was far from being matched. For the Romans, although fighting far from their country, had been easily persuaded by their generals that they were fighting in defence of Italy and the city of Rome. And so, as men whose return to their native land would depend upon the issue of that battle, they had made up their [p. 99]minds to win or die. [8] The other battle-line had men² less firmly resolved. For the majority were Spaniards, who preferred to be vanquished in Spain, rather than as victors to be dragged to Italy. [9] Therefore at the first clash, when they had barely hurled their javelins, the centre fell back, and, as the Romans advanced with a great charge, retreated. On the wrings, however, there was more spirited fighting. [10] On the one hand the Carthaginians pressed them hard, on the other hand the Africans; and it was a double conflict against men presumed to have been surrounded. [11] But, although the whole Roman line had by this time crowded into the centre, it had sufficient strength to force apart the wings of the enemy. [12] Thus there were two battles in opposite directions. In both the Romans were unquestionably victorious, since, once the centre had been routed, they were superior both in the numbers and in the strength of their men. [13] A great number of men³ were slain there, and if the Spaniards had not fled in such confusion when the battle had scarcely begun, very few out of that entire line would have survived. [14] The cavalry were not engaged at all, since, as soon as the Mauri and the Numidians saw the centre giving way, they at once abandoned the wings, exposed by their wild flight as they drove the elephants also before them. [15] Hasdrubal, after waiting for the final outcome of the battle, escaped with a few men out of the midst of the slaughter. His camp the Romans captured and plundered. [16] That battle brought to the Roman side all that still wavered in Spain, and Hasdrubal had left to him no hope, not only of leading his army over into Italy, but not even of remaining with any safety in Spain. [17] When these facts were generally known [p. 101]at Rome through the letter of the Scipios, people⁴ rejoiced, not so much over the victory, as that Hasdrubal's crossing into Italy had been prevented.⁵