Titus Livius (Livy), The History of Rome, Book 21
Benjamin Oliver Foster, Ph.D., Ed.

55. Hannibal's soldiers had in the meantime made fires before their tents; in each company they had been served with oil to supple their joints, and had breakfasted at leisure. When, therefore, they were told that the enemy had crossed the river, they were eager both in mind and body, as they armed and went out to battle. [2] In front of the standards Hannibal placed the Baliares, light-armed troops numbering about eight thousand, and behind these his heavy infantry, tile strength and flower of his army; the wings he formed of ten thousand horse, and, dividing the elephants, stationed them outside the wings. [3] The consul's troopers were scattered in pursuit of the Numidians, when suddenly the latter made a stand and took them unawares; whereupon he called them back and posted them on either flank of the infantry. [4] There were eighteen thousand Romans and twenty thousand allies of the Latin name, besides the auxiliaries of the Cenomani, the only Gallic tribe that continued loyal. These were the contending forces.

The Baliares began the battle, but those light-armed troops, finding the legions too strong to cope with, were quickly withdrawn and sent to the wings. [5] This manœuvre at once caused the Roman cavalry acute distress; for they numbered but four thousand, and, tired as they were, would scarce have been able to hold out any longer against the enemy's ten [p. 165]thousand cavalry alone, who were most of them¹ fresh; [6] and now they were overwhelmed, as it were with a cloud of missiles, by the Baliares. [7] Besides this, the elephants, looming large on the outer extremities of the wings, gave rise to such a panic, particularly among the horses, not only by their strange appearance, but also by their unfamiliar smell, as to bring about a general flight. [8] As for the infantry, they were fairly matched in courage, but not in strength, which was unimpaired in the case of the Phoenicians, who had refreshed themselves shortly before entering the battle, while the Romans were faint with fasting and fatigue, and were stiff and numb with cold. Yet their courage would have enabled them to resist, had they fought against infantry alone. [9] But the Baliares, having put the cavalry to flight, were raining missiles on their flanks; the elephants had now charged the centre of the line; and Mago and his Numidians, as soon as the Roman army had passed their ambuscade without observing it, started up in their rear, and caused the wildest panic and confusion. [10] Nevertheless, amidst all these evils, the line held for a time unshaken, and even —what no one had dared to hope for —against the elephants. [11] Skirmishers, expressly posted to deal with the beasts, would throw darts at them and make them turn away, and then pursuing them would strike them under the tail, where the skin is softest and it is possible to wound them.