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.