Publius, having crossed the Po and encamped at Placentia, a Roman colony, where he occupied himself with the cure of himself and the other wounded, and thinking that his forces were now firmly established in a safe position, made no move. But two days after his crossing Hannibal appeared close at hand and next day drew up his army in full view of the enemy. Upon their refusing his challenge, he encamped at a distance of about fifty stades from the Roman position.
The Celtic contingents in the Roman army, seeing that the prospects of the Carthaginians were now brighter, had come to an understanding with each other, and while all remaining quiet in their tents were waiting for an opportunity to attack the Romans. All in the entrenched camp had had their supper and retired to rest, and the Celts, letting the greater part of the night go by, armed themselves about the morning watch and fell upon the Romans who were encamped nearest to them. They killed or wounded many, and finally, cutting off the heads of the slain, went over to the Carthaginians, being in number about two thousand foot and rather less than two hundred horse. They were gladly welcomed on their arrival by Hannibal, who at once, after addressing some words of encouragement to them and promising suitable gifts to all, sent them off to their own cities to announce to their countrymen what they had done and urge them to join him. For he was now quite sure that all would take his part on learning of this act of treachery to the Romans on the part of their own countrymen. When at the same time the Boii came to him and delivered up to him the three Roman officials charged with the partition of their lands, whom, as I mentioned above, they had originally captured by treachery, Hannibal welcomed their friendly advances and made a formal alliance with them through the envoys. He gave the three Romans, however, back to them, advising them to keep them in order through them to get their own hostages back, as had been their original design.
Publius was much concerned at this act of treachery, and taking into consideration that as the Celts had been disaffected for some time, now with this additional incentive all the Gauls round about would go over to the Carthaginians, decided to take precautions for the future. In consequence he broke up his camp that same night a little before daybreak and marched towards the river Trebia and the hills in its neighbourhood, relying on the natural strength of the country and the loyalty of the neighbouring allies.
Publius, crossing the Trebia, encamped on the first hills he reached and fortifying his camp with a trench and palisade awaited the arrival of Tiberius and his forces. In the meantime he attended carefully to the treatment of his wound, as he was anxious to be able to take part in the coming battle. Hannibal encamped at a distance of about forty stades from the enemy. The numerous Celtic population of the plain, enthusiastically taking up the cause of the Carthaginians, kept the camp furnished with abundance of provisions and were ready to take their part in any of Hannibal’s operations or battles.
At about the same time the town of Clastidium was betrayed to Hannibal by a native of Brundisium, to whom the Romans had entrusted it, the garrison and all the stores of grain falling into his hands. The latter he used for his present needs, but he took the men he had captured with him without doing them any hurt, wishing to make a display of leniency, so that those who were overtaken by adversity should not be terrified and give up hope of their lives being spared by him. He conferred high honours on the traitor, as he was anxious to win over those in positions of authority to the Carthaginian cause.
After this, on observing that some of the Celts who lived between the Trebia and the Po had made alliance with himself, but were negotiating with the Romans also, under the idea that thus they would be safe from both, he dispatched two thousand foot and about a thousand Celtic and Numidian horse with orders to raid their country. On his orders being executed and a large amount of booty secured, the Celts at once came into the Roman camp asking for help.
(Source: Polybius, The Histories, Vol.II, Book III, Loeb Classical Library)
Research-Selection for NovoScriptorium: Anastasius Philoponus