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.

But the Roman infantry proved itself worthy of its name: at the beginning of the battle it fought with very decided superiority against the infantry of the enemy, and even when the repulse of the Roman horse allowed the enemy's cavalry and light-armed troops to turn their attacks against the Roman infantry, the latter, although ceasing to advance, obstinately maintained its ground. At this stage a select Carthaginian band of 1000 infantry, and as many horsemen, under the leadership of Mago, Hannibal's youngest brother, suddenly emerged from an ambush in the rear of the Roman army, and fell upon the densely entangled masses. The wings of the army and the rear ranks of the Roman centre were broken up and scattered by this attack, while the first division, 10,000 men strong, in compact array broke through the Carthaginian line, and made a passage for itself obliquely through the midst of the enemy, inflicting great loss on the opposing infantry and more especially on the Gallic insurgents. This brave body, pursued but feebly, thus reached Placentia. The remaining mass was for the most part slaughtered by the elephants and light troops of the enemy in attempting to cross the river: only part of the cavalry and some divisions of infantry were able, by wading through the river, to gain the camp whither the Carthaginians did not follow them, and thus they too reached Placentia[1]. Few battles confer more honour on the Roman soldier than this on the Trebia, and few at the same time furnish graver impeachment of the general in command; although the candid judge will not forget that a commandership in chief expiring on a definite day was an unmilitary institution, and that figs cannot be reaped from thistles. The victory came to be costly even to the victors. Although the loss in the battle fell chiefly on the Celtic insurgents, yet a multitude of the veteran soldiers of Hannibal died afterwards from diseases engendered by that raw and wet winter day, and all the elephants perished except one.

The effect of this first victory of the invading army was, that the national insurrection now spread and assumed shape without hindrance throughout the Celtic territory. The remains of the Roman army of the Po threw themselves into the fortresses of Placentia and Cremona: completely cut off from home, they were obliged to procure their supplies by way of the river. The consul Tiberius Sempronius only escaped, as if by miracle, from being taken prisoner, when with a weak escort of cavalry he went to Rome on account of the elections. Hannibal, who would not hazard the health of his troops by further marches at that inclement season, bivouacked for the winter where he was; and, as a serious attempt on the larger fortresses would have led to no result, contented himself with annoying the enemy by attacks on the river port of Placentia and other minor Roman positions. He employed himself mainly in organizing the Gallic insurrection: more than 60,000 foot soldiers and 4000 horsemen from the Celts are said to have joined his army.

No extraordinary exertions were made in Rome for the campaign of 537. The senate thought, and not unreasonably, that, despite the lost battle, their position was by no means fraught with serious danger. Besides the coast garrisons, which were despatched to Sardinia, Sicily, and Tarentum, and the reinforcements which were sent to Spain, the two new consuls Gaius Flaminius and Gnaeus Servilius obtained only as many men as were necessary to restore the four legions to their full complement; additions were made to the strength of the cavalry alone. The consuls had to protect the northern frontier, and stationed themselves accordingly on the two highways which led from Rome to the north, the western of which at that lime terminated at Arretium, and the eastern at Ariminum; Gaius Flaminius occupied the former, Gnaeus Servilius the latter. There they ordered the troops from the fortresses on the Po to join them, probably by water, and awaited the commencement of the favourable season, when they proposed to occupy in the defensive the passes of the Apennines, and then, taking up the offensive, to descend into the valley of the Po and effect a junction somewhere near Placentia. But Hannibal by no means intended to defend the valley of the Po. He knew Rome better perhaps than the Romans knew it themselves, and was very well aware how decidedly he was the weaker and continued to be so notwithstanding the brilliant battle on the Trebia; he knew too that his ultimate object, the humiliation of Rome, was not to be wrung from the unbending Roman pride either by terror or by surprise, but could only be gained by the actual subjugation of the haughty city. It was clearly apparent that the Italian federation was in political solidity and in military resources infinitely superior to an adversary, who received only precarious and irregular support from home, and who in Italy was dependent for primary aid solely on the vacillating and capricious nation of the Celts; and that the Phoenician foot soldier was, notwithstanding all the pains taken by Hannibal, far inferior in point of tactics to the legionary, had been completely proved by the defensive movements of Scipio and the brilliant retreat of the defeated infantry on the Trebia. From this conviction flowed the two fundamental principles which determined Hannibal's whole method of operations in Italy - viz., that the war should be carried on, in somewhat adventurous fashion, with constant changes in the plan and in the theatre of operations; and that its favourable issue could only be looked for as the result of political and not of military successes - of the gradual loosening and final breaking up of the Italian federation. That mode of carrying on the war was necessary, because the single element which Hannibal had to throw into the scale against so many disadvantages - his military genius - only told with its full weight, when he constantly foiled his opponents by unexpected combinations; he was undone, if the war became stationary. That aim was the aim dictated to him by right policy, because, mighty conqueror though he was in battle, he saw very clearly that on each occasion he vanquished the generals and not the city, and that after each new battle the Romans remained just as superior to the Carthaginians as he was personally superior to the Roman commanders. That Hannibal even at the height of his fortune never deceived himself on this point, is worthier of admiration than his most admired battles.