Hannibal pitched his camp on the Nure, about five miles from Piacenza, but Scipio remained immovable in his lines waiting for the arrival of his colleague. Hannibal's position was a difficult one. He had traversed the Pyrenees and the Alps that he might attack Rome; but between him and Southern Italy lay yet another barrier, the Apennines. Scipio had missed him after he had crossed the Pyrenees, had been too late to attack him when, exhausted and worn out, his army emerged from the Alps; but now, united with Sempronius, he hoped to crush him at the foot of the Apennines. Hannibal wished, if possible, to prevent a junction of the two Roman armies, but if that could not be done he determined to fight them together.
Scipio perceived the danger of his position; and in order to be able the better to join Sempronius he left Piacenza under cover of night, and took up a strong position on the banks of the Trebia. Here he could maintain his communications direct with Rome, and, if absolutely necessary, fall back and join his colleague advancing towards him. Hannibal, when he perceived Scipio's change of position, broke up his camp and took post on the Trebiola, a little stream running into the Trebia and facing the Roman camp at a distance of four miles.
He was now powerless to prevent the junction of the two Roman armies, and for nearly a month Scipio and Hannibal lay watching each other. By that time Sempronius was within a day's march of Scipio. Hannibal had not been idle during this time of rest. He had been occupied in cementing his alliance with the Gaulish tribes inhabiting the Lombard plains. These, seeing how rapidly Hannibal had cleared the province of the Romans, believed that their deliverance would be accomplished, and for the most part declared for the Carthaginians.
Hannibal's agents had also been at work at Clastidium, and the prefect of the garrison was induced by a bribe to surrender the place to him. This was of enormous advantage to Hannibal, and a corresponding blow to the Romans, for Clastidium was the chief magazine north of the Apennines. The news of the fall of this important place filled Sempronius, an energetic and vigorous general, with fury. He at once rode down from his camp to that of Scipio and proposed that Hannibal should be attacked instantly.
Scipio, who was still suffering from the wound he had received in the cavalry engagement, urged that the Roman army should remain where they were, if necessary, through the coming winter. He pointed out that Hannibal's Gaulish allies would lose heart at seeing him inactive, and would cease to furnish him with supplies, and that he would be obliged either to attack them at a disadvantage or to retire from the position he occupied. But Sempronius was an ambitious man, the time for the consular election was approaching, and he was unwilling to leave for his successor the glory of crushing Hannibal.
The fact, too, that Scipio was wounded and unable to take part in the battle added to his desire to force it on, since the whole glory of the victory would be his. He therefore told his colleague that although he saw the force of his arguments, public opinion in Rome was already so excited at Hannibal having been allowed, without a battle, to wrest so wide a territory from Rome, that it was absolutely necessary that an action should be fought. The two armies were now united on the Trebia, and opinion was among the officers and troops, as between the consuls, widely divided as to the best course to be pursued.
Hannibal's spies among the natives kept him acquainted with what was going on in the Roman camp, and he determined to provoke the Romans to battle. He therefore despatched two thousand infantry and a thousand cavalry to ravage the lands of some Gaulish allies of the Romans. Sempronius sent off the greater part of his cavalry, with a thousand light infantry, to drive back the Carthaginians.
In the fight which ensued the Romans were worsted. Still more furious, Sempronius marched to support them with his army. Hannibal called in his troops and drew them off before Sempronius would arrive. The disappointment and rage of the Roman general were great, and Hannibal felt that he could now bring on a battle when he would. He determined to fight in the plain close to his own position. This was flat and bare, and was traversed by the Trebiola. This stream ran between steep banks below the level of the plain; its banks were covered with thick bushes and reeds, and the narrow gap across the plain was scarce noticeable.
On the evening of the twenty-fifth of December Hannibal moved his army out from the camp and formed up on the plain facing the Trebia, ordering the corps commanded by his brother Mago to enter the bed of the Trebiola, and to conceal themselves there until they received his orders to attack. The position Mago occupied would bring him on the left rear of an army which had crossed the Trebia, and was advancing to attack the position taken up by Hannibal. Having thus prepared for the battle, Hannibal proceeded to provoke it.
At daybreak on the twenty-sixth he despatched a strong body of horsemen across the river. Crossing the Trebia partly by ford and partly by swimming, the Carthaginian horse rode up to the palisade surrounding the Roman camp, where, with insulting shouts and the hurling of their javelins, they aroused the Roman soldiers from their slumber. This insult had the desired effect, Sempronius rushed from his tent, furious at what he deemed the insolence of the Carthaginians, and called his troops to arms. With their accustomed discipline the Romans fell into their ranks. The light cavalry first issued from the palisade, the infantry followed, the heavy cavalry brought up the rear. The insulting Numidians had already retired, but Sempronius was now determined to bring on the battle. He marched down the river and crossed at a ford.
The water was intensely cold, the river was in flood, the ford waist deep as the soldiers marched across it. Having gained the opposite bank, the Roman general formed his army in order of battle. His infantry, about forty-five thousand strong, was formed in three parallel lines; the cavalry, five thousand strong, was on the flanks. The infantry consisted of sixteen thousand Roman legionary or heavy infantry, and six thousand light infantry. The Italian tribes, allied to Rome, had supplied twenty thousand infantry; the remaining three thousand were native allies. The infantry occupied a front of two and a half miles in length; the cavalry extended a mile and a quarter on each flank. Thus the Roman front of battle was five miles in extent.
Hannibal's force was inferior in strength; his infantry of the line were twenty thousand strong. He had eight thousand light infantry and ten thousand cavalry. The Carthaginian formation was much deeper than the Roman, and Hannibal's line of battle was less than two miles long. In front of it were the elephants, thirty-six in number, divided in pairs, and placed in intervals of a hundred yards between each pair.
While the Romans, exposed to a bitterly cold wind, chilled to the bone by their immersion in the stream, and having come breakfastless from camp, were forming their long order of battle, Hannibal's troops, gathered round blazing fires, were eating a hearty breakfast; after which, in high spirits and confidence, they prepared for the fight.
Hannibal called the officers together and addressed them in stirring words, which were repeated by them to the soldiers. The Roman preparations had occupied a long time, and it was afternoon before they advanced in order of battle. When within a short distance of the Carthaginians they halted, and the trumpets and musical instruments on both sides blew notes of defiance. Then the Carthaginian slingers stole out between the ranks of their heavy infantry, passed between the elephants, and commenced the battle.