Scipio, who, with an army considerably smaller and very weak in cavalry, had the difficult task of preventing the advance of the superior force of the enemy and of repressing the movements of insurrection which everywhere were spreading among the Celts, had crossed the Po presumably at Placentia, and marched up the river to meet the enemy, while Hannibal after the capture of Turin marched downwards to relieve the Insubres and Boii. In the plain between the Ticino and the Sesia, not far from Vercelli, the Roman cavalry, which had advanced with the light infantry to make a reconnaissance in force, encountered the Punic cavalry sent out for the like purpose, both led by the generals in person. Scipio accepted battle when offered, notwithstanding the superiority of the enemy; but his light infantry, which was placed in front of the cavalry, dispersed before the charge of the heavy cavalry of the enemy, and while the latter engaged the masses of the Roman horsemen in front, the light Numidian cavalry, after having pushed aside the broken ranks of the enemy's infantry, took the Roman horsemen in flank and rear. This decided the combat. The loss of the Romans was very considerable. The consul himself, who made up as a soldier for his deficiencies as a general, received a dangerous wound, and owed his safety entirely to the devotion of his son of seventeen, who, courageously dashing into the ranks of the enemy, compelled his squadron to follow him and rescued his father. Scipio, enlightened by this combat as to the strength of the enemy, saw the error which he had committed in posting himself, with a weaker army, in the plain with his back to the river, and resolved to return to the right bank of the Po under the eyes of his antagonist. As the operations became contracted into a narrower space and his illusions regarding Roman invincibility departed, he recovered the use of his considerable military talents, which the adventurous boldness of his youthful opponent's plans had for a moment paralyzed. While Hannibal was preparing for a pitched battle, Scipio by a rapidly projected and steadily executed march succeeded in reaching the right bank of the river which in an evil hour he had abandoned, and broke down the bridge over the Po behind his army; the Roman detachment of 600 men charged to cover the process of destruction were, however, intercepted and made prisoners. But as the upper course of the river was in the hands of Hannibal, he could not be prevented from marching up the stream, crossing on a bridge of boats, and in a few days confronting the Roman army on the right bank. The latter had taken a position in the plain in front of Placentia; but the mutiny of a Celtic division in the Roman camp, and the Gallic insurrection breaking out afresh all around, compelled the consul to evacuate the plain and to post himself on the hills behind the Trebia. This was accomplished without notable loss, because the Numidian horsemen sent in pursuit lost their time in plundering, and setting fire to, the abandoned camp. In this strong position, with his left wing resting on the Apennines, his right on the Po and the fortress of Placentia, and covered in front by the Trebia - no inconsiderable stream at that season - Scipio was unable to save the rich stores of Clastidium (Casteggio) from which in this position he was cut off by the army of the enemy; nor was he able to avert the insurrectionary movement on the part of almost all the Gallic cantons, excepting the Cenomani who were friendly to Rome; but he completely checked the progress of Hannibal, and compelled him to pitch his camp opposite to that of the Romans. Moreover, the position taken up by Scipio, and the circumstance of the Cenomani threatening the borders of the Insubres, hindered the main body of the Gallic insurgents from directly joining the enemy, and gave to the second Roman army, which meanwhile had arrived at Ariminum from Lilybaeum, the opportunity of reaching Placentia through the midst of the insurgent country without material hindrance, and of uniting itself with the army of the Po.
Scipio had thus solved his difficult task completely and brilliantly. The Roman army, now close on 40,000 strong, and though not a match for its antagonist in cavalry, at least equal in infantry, had simply to remain in its existing position, in order to compel the enemy either to attempt in the winter season the passage of the river and an attack upon the camp, or to suspend his advance and to test the fickle temper of the Gauls by the burden of winter quarters. Clear, however, as this was, it was no less clear that it was now December, and that under the course proposed the victory might perhaps be gained by Rome, but would not be gained by the consul Tiberius Sempronius, who held the sole command in consequence of Scipio's wound, and whose year of office expired in a few months. Hannibal knew the man, and neglected no means of alluring him to fight. The Celtic villages that had remained faithful to the Romans were cruelly laid waste, and, when this brought on a conflict between the cavalry, Hannibal allowed his opponents to boast of the victory. Soon thereafter on a raw rainy day a general engagement came on, unlocked for by the Romans. From the earliest hour of the morning the Roman light troops had been skirmishing with the light cavalry of the enemy; the latter slowly retreated, and the Romans eagerly pursued it through the deeply swollen Trebia to follow up the advantage which they had gained. Suddenly the cavalry halted; the Roman vanguard found itself face to face with the army of Hannibal drawn up for battle on a field chosen by himself; it was lost, unless the main body should cross the stream with all speed to its support. Hungry, weary, and wet, the Romans came on and hastened to form in order of battle, the cavalry, as usual, on the wings, the infantry in the centre. The light troops, who formed the vanguard on both sides, began the combat: but the Romans had already almost exhausted their missiles against the cavalry, and immediately gave way. In like manner the cavalry gave way on the wings, hard pressed by the elephants in front, and outflanked right and left by the far more numerous Carthaginian horse.