The object was attained, but at a heavy cost. Of the 50,000 veteran infantry and the 9000 cavalry, which the army had numbered at the crossing of the Pyrenees, more than half had been sacrificed in the conflicts, the marches, and the passages of the rivers. Hannibal now, according to his own statement, numbered not more than 20,000 infantry - of whom three-fifths were Libyans and two-fifths Spaniards - and 6000 cavalry, part of them doubtless dismounted: the comparatively small loss of the latter proclaimed the excellence of the Numidian cavalry no less than the consideration of the general in making a sparing use of troops so select. A march of 526 miles or about 33 moderate days' marching - the continuance and termination of which were disturbed by no special misfortunes on a great scale that could not be anticipated, but were, on the other hand, rendered possible only by incalculable pieces of good fortune and still more incalculable blunders of the enemy, and which yet not only cost such sacrifices, but so fatigued and demoralized the army, that it needed a prolonged rest in order to be again ready for action - is a military operation of doubtful value, and it may be questioned whether Hannibal himself regarded it as successful. Only in so speaking we may not pronounce an absolute censure on the general: we see well the defects of the plan of operations pursued by him, but we cannot determine whether he was in a position to foresee them - his route lay through an unknown land of barbarians - or whether any other plan, such as that of taking the coast road or of embarking at Cartagena or at Carthage, would have exposed him to fewer dangers. The cautious and masterly execution of the plan in its details at any rate deserves our admiration, and to whatever causes the result may have been due - whether it was due mainly to the favour of fortune, or mainly to the skill of the general - the grand idea of Hamilcar, that of taking up the conflict with Rome in Italy, was now realized. It was his genius that projected this expedition; and as the task of Stein and Scharnhorst was more difficult and nobler than that of York and Blucher, so the unerring tact of historical tradition has always dwelt on the last link in the great chain of preparatory steps, the passage of the Alps, with a greater admiration than on the battles of the Trasimene lake and of the plain of Cannae.

The appearance of the Carthaginian army on the Roman side of the Alps changed all at once the situation of affairs, and disconcerted the Roman plan of war. Of the two principal armies of the Romans, one had landed in Spain and was already engaged with the enemy there: it was no longer possible to recall it. The second, which was destined for Africa under the command of the consul Tiberius Sempronius, was fortunately still in Sicily: in this instance Roman delay for once proved useful. Of the two Carthaginian squadrons destined for Italy and Sicily, the first was dispersed by a storm, and some of its vessels were captured by the Syracusans near Messana; the second had endeavoured in vain to surprise Lilybaeum, and had thereafter been defeated in a naval engagement off that port. But the continuance of the enemy's squadrons in the Italian waters was so inconvenient, that the consul determined, before crossing to Africa, to occupy the small islands around Sicily, and to drive away the Carthaginian fleet operating against Italy. The summer passed away in the conquest of Melita, in the chase after the enemy's squadron, which he expected to find at the Lipari islands while it had made a descent near Vibo (Monteleone) and pillaged the Bruttian coast, and, lastly, in gaining information as to a suitable spot for landing on the coast of Africa; so that the army and fleet were still at Lilybaeum, when orders arrived from the senate that they should return with all possible speed for the defence of their homes.

In this way, while the two great Roman armies, each in itself equal in numbers to that of Hannibal, remained at a great distance from the valley of the Po, the Romans were quite unprepared for an attack in that quarter. No doubt a Roman army was there, in consequence of an insurrection that had broken out among the Celts even before the arrival of the Carthaginian army. The founding of the two Roman strongholds of Placentia and Cremona, each of which received 6000 colonists, and more especially the preparations for the founding of Mutina in the territory of the Boii, had already in the spring of 536 driven the Boii to revolt before the time concerted with Hannibal; and the Insubres had immediately joined them. The colonists already settled in the territory of Mutina, suddenly attacked, took refuge in the town. The praetor Lucius Manlius, who held the chief command at Ariminum, hastened with his single legion to relieve the blockaded colonists; but he was surprised in the woods, and no course was left to him after sustaining great loss but to establish himself upon a hill and to submit to a siege there on the part of the Boii, till a second legion sent from Rome under the praetor Lucius Atilius succeeded in relieving army and town, and in suppressing for the moment the Gaulish insurrection. This premature rising of the Boii on the one hand, by delaying the departure of Scipio for Spain, essentially promoted the plans of Hannibal; on the other hand, but for its occurrence he would have found the valley of the Po entirely unoccupied, except the fortresses. But the Roman corps, whose two severely thinned legions did not number 20,000 soldiers, had enough to do to keep the Celts in check, and did not think of occupying the passes of the Alps. The Romans only learned that the passes were threatened, when in August the consul Publius Scipio returned without his army from Massilia to Italy, and perhaps even then they gave little heed to the matter, because, forsooth, the foolhardy attempt would be frustrated by the Alps alone. Thus at the decisive hour and on the decisive spot there was not even a Roman outpost. Hannibal had full time to rest his army, to capture after a three days' siege the capital of the Taurini which closed its gates against him, and to induce or terrify into alliance with him all the Ligurian and Celtic communities in the upper basin of the Po, before Scipio, who had taken the command in the Po valley, encountered him.

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.