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

54. Between the two camps was a water-course, shut in by very high banks on either side and overgrown all round with marsh-grass and the underbrush and brambles with which uncultivated land is usually clothed. When Hannibal, riding over the ground himself, saw that this place afforded sufficient cover even for cavalry, he said to his brother Mago, “This will be the place for you to hold. [2] Choose out a hundred men from all the infantry and a hundred from the cavalry, and come with them to my quarters at the first watch. It is time now to sup and rest.” With that he broke up the council. In a little while Mago presented himself with his picked men. [3] “I see the stoutest of my men,” said [p. 161]Hannibal, “but that your numbers too may be strong to¹ match your bravery, choose, each of you, from the squadrons and the maniples, nine others like yourselves. Mago will point out to you the spot where you are to lie in ambush; you have an enemy who is blind to these stratagems.” [4] Mago and his thousand horse and thousand foot being thus dispatched, Hannibal ordered the Numidian cavalry to cross the Trebia at dawn, and riding up to the enemy's gates and discharging missiles against his outposts, to lure him into battle; and then, when the fight was on, to give ground insensibly and draw him across the river. Such were the orders of the Numidians. [5] The other officers, both of cavalry and of infantry, were instructed to make their men have breakfast, and then, armed and with horses saddled, to await the signal.

[6] On the flurry caused by the Numidians, Sempronius, confident where cavalry was concerned, first led out all of this part of his forces; then six thousand of the infantry; and finally all the rest of his troops. He had fully made up his mind beforehand and was eager for the battle. [7] It chanced to be the time of year when the days are shortest, and it was snowing in the region between the Alps and the Apennines, and the proximity of rivers and marshes intensified the bitter cold. [8] Moreover, men and horses had been turned out in haste, without stopping for food or doing anything to guard against becoming chilled; there was no warmth in them, and the nearer they approached the atmosphere of the river the sharper grew the cold wind in their faces. [9] But when, in pursuit of the fleeing Numidians, they entered the water —swollen breast-high with the [p. 163]rain that had fallen in the night —or at any rate² when they got out upon the further bank, then indeed their bodies were all so benumbed that they could hardly hold their weapons; and at the same time they were fainting with fatigue, and, as the day wore on, with hunger as well.