The condition and prospects of both the Roman and Carthaginian cause were entirely changed by this battle, and the retreat of Scipio across the Po. All the nations of the north of Italy, who had been subjects or allies of the Romans, now turned to Hannibal. They sent embassies into his camp, offering him their friendship and alliance. In fact, there was a large body of Gauls in the Roman camp, who were fighting under Scipio at the battle of Ticinus, who deserted his standard immediately afterward, and came over in a mass to Hannibal. They made this revolt in the night, and, instead of stealing away secretly, they raised a prodigious tumult, killed the guards, filled the encampment with their shouts and outcries, and created for a time an awful scene of terror.

Hannibal received them, but he was too sagacious to admit such a treacherous horde into his army. He treated them with great consideration and kindness, and dismissed them with presents, that they might all go to their respective homes, charging them to exert their influence in his favor among the tribes to which they severally belonged.

Hannibal's soldiers, too, were very much encouraged by the commencement they had made. The army made immediate preparations for crossing the river. Some of the soldiers built rafts, others went up the stream in search of places to ford. Some swam across. They could adopt these or any other modes in safety, for the Romans made no stand on the opposite bank to oppose them, but moved rapidly on, as fast as Scipio could be carried. His wounds began to inflame, and were extremely painful.

In fact, the Romans were dismayed at the danger which now threatened them. As soon as news of these events reached the city, the authorities there sent a dispatch immediately to Sicily to recall the other consul. His name was Sempronius. It will be recollected that, when the lots were cast between him and Scipio, it fell to Scipio to proceed to Spain, with a view to arresting Hannibal's march, while Sempronius went to Sicily and Africa. The object of this movement was to threaten and attack the Carthaginians at home, in order to distract their attention and prevent their sending any fresh forces to aid Hannibal, and, perhaps, even to compel them to recall him from Italy to defend their own capital. But now that Hannibal had not only passed the Alps, but had also crossed the Po, and was marching toward Rome—Scipio himself disabled, and his army flying before him—they were obliged at once to abandon the plan of threatening Carthage. They sent with all dispatch an order to Sempronius to hasten home and assist in the defense of Rome.

Sempronius was a man of a very prompt and impetuous character, with great confidence in his own powers, and very ready for action. He came immediately into Italy, recruited new soldiers for the army, put himself at the head of his forces, and marched northward to join Scipio in the valley of the Po. Scipio was suffering great pain from his wounds, and could do but little toward directing the operations of the army. He had slowly retreated before Hannibal, the fever and pain of his wounds being greatly exasperated by the motion of traveling. In this manner he arrived at the Trebia, a small stream flowing northward into the Po. He crossed this stream, and finding that he could not go any further, on account of the torturing pain to which it put him to be moved, he halted his army, marked out an encampment, threw up fortifications around it, and prepared to make a stand. To his great relief, Sempronius soon came up and joined him here.

There were now two generals. Napoleon used to say that one bad commander was better than two good ones, so essential is it to success in all military operations to secure that promptness, and confidence, and decision which can only exist where action is directed by one single mind. Sempronius and Scipio disagreed as to the proper course to be pursued. Sempronius wished to attack Hannibal immediately. Scipio was in favor of delay. Sempronius attributed Scipio's reluctance to give battle to the dejection of mind and discouragement produced by his wound, or to a feeling of envy lest he, Sempronius, should have the honor of conquering the Carthaginians, while he himself was helpless in his tent. On the other hand, Scipio thought Sempronius inconsiderate and reckless; and disposed to rush heedlessly into a contest with a foe whose powers and resources he did not understand.

In the mean time, while the two commanders were thus divided in opinion, some skirmishes and small engagements took place between detachments from the two armies, in which Sempronius thought that the Romans had the advantage. This excited his enthusiasm more and more, and he became extremely desirous to bring on a general battle. He began to be quite out of patience with Scipio's caution and delay. The soldiers, he said, were full of strength and courage, all eager for the combat, and it was absurd to hold them back on account of the feebleness of one sick man. "Besides," said he, "of what use can it be to delay any longer? We are as ready to meet the Carthaginians now as we shall ever be. There is no third  consul to come and help us; and what a disgrace it is for us Romans, who in the former war led our troops to the very gates of Carthage, to allow Hannibal to bear sway over all the north of Italy, while we retreat gradually before him, afraid to encounter now a force that we have always conquered before."

Hannibal was not long in learning, through his spies, that there was this difference of opinion between the Roman generals, and that Sempronius was full of a presumptuous sort of ardor, and he began to think that he could contrive some plan to draw the latter out into battle under circumstances in which he would have to act at a great disadvantage. He did contrive such a plan. It succeeded admirably; and the case was one of those numerous instances which occurred in the history of Hannibal, of successful stratagem, which led the Romans to say that his leading traits of character were treachery and cunning.

Hannibal's plan was, in a word, an attempt to draw the Roman army out of its encampment on a dark, cold, and stormy night in December, and get them into the river. This river was the Trebia. It flowed north into the Po, between the Roman and Carthaginian camps. His scheme, in detail, was to send a part of his army over the river to attack the Romans in the night or very early in the morning. He hoped that by this means Sempronius would be induced to come out of his camp to attack the Carthaginians. The Carthaginians were then to fly and recross the river, and Hannibal hoped that Sempronius would follow, excited by the ardor of pursuit. Hannibal was then to have a strong reserve of the army, that had remained all the time in warmth and safety, to come out and attack the Romans with unimpaired strength and vigor, while the Romans themselves would be benumbed by the cold and wet, and disorganized by the confusion produced in crossing the stream.

A part of Hannibal's reserve were to be placed in an ambuscade. There were some meadows near the water, which were covered in many places with tall grass and bushes. Hannibal went to examine the spot, and found that this shrubbery was high enough for even horsemen to be concealed in it. He determined to place a thousand foot soldiers and a thousand horsemen here, the most efficient and in the army. He selected them in the following manner:

He called one of his lieutenant generals to the spot, explained somewhat of his design to him, and then asked him to go and choose from the cavalry and the infantry, a hundred each, the best soldiers he could find. This two hundred were then assembled, and Hannibal, after surveying them with looks of approbation and pleasure, said, "Yes, you are the men I want, only, instead of two hundred, I need two thousand. Go back to the army, and select and bring to me, each of you, nine men like yourselves." It is easy to be imagined that the soldiers were pleased with this commission, and that they executed it faithfully. The whole force thus chosen was soon assembled, and stationed in the thickets above described, where they lay in ambush ready to attack the Romans after they should pass the river.

Hannibal also made arrangements for leaving a large part of his army in his own camp, ready for battle, with orders that they should partake of food and refreshments, and keep themselves warm by the fires until they should be called upon. All things being thus ready, he detached a body of horsemen to cross the river, and see if they could provoke the Romans to come out of their camp and pursue them.

"Go," said Hannibal, to the commander of this detachment, "pass the stream, advance to the Roman camp, assail the guards, and when the army forms and comes out to attack you, retreat slowly before them back across the river."

The detachment did as it was ordered to do. When they arrived at the camp, which was soon after break of day—for it was a part of Hannibal's plan to bring the Romans out before they should have had time to breakfast—Sempronius, at the first alarm, called all the soldiers to arms, supposing that the whole Carthaginian force was attacking them. It was a cold and stormy morning, and the atmosphere being filled with rain and snow, but little could be seen. Column after column of horsemen and of infantry marched out of the camp. The Carthaginians retreated. Sempronius was greatly excited at the idea of so easily driving back the assailants, and, as they retreated, he pressed on in pursuit of them. As Hannibal had anticipated, he became so excited in the pursuit that he did not stop at the banks of the river. The Carthaginian horsemen plunged into the stream in their retreat, and the Romans, foot soldiers and horsemen together, followed on. The stream was usually small, but it was now swelled by the rain which had been falling all the night. The water was, of course, intensely cold. The horsemen got through tolerably well, but the foot soldiers were all thoroughly drenched and benumbed; and as they had not taken any food that morning, and had come forth on a very sudden call, and without any sufficient preparation, they felt the effects of the exposure in the strongest degree. Still they pressed on. They ascended the bank after crossing the river, and when they had formed again there, and were moving forward in pursuit of their still flying enemy, suddenly the whole force of Hannibal's reserves, strong and vigorous, just from their tent and their fires, burst upon them. They had scarcely recovered from the astonishment and the shock of this unexpected onset, when the two thousand concealed in the ambuscade came sallying forth in the storm, and assailed the Romans in the rear with frightful shouts and outcries.

All these movements took place very rapidly. Only a very short period elapsed from the time that the Roman army, officers and soldiers, were quietly sleeping in their camp, or rising slowly to prepare for the routine of an ordinary day, before they found themselves all drawn out in battle array some miles from their encampment, and surrounded and hemmed in by their foes. The events succeeded each other so rapidly as to appear to the soldiers like a dream; but very soon their wet and freezing clothes, their limbs benumbed and stiffened, the sleet which was driving along the plain, the endless lines of Carthaginian infantry, hemming them in on all sides, and the columns of horsemen and of elephants charging upon them, convinced them that their situation was one of dreadful reality. The calamity, too, which threatened them was of vast extent, as well as imminent and terrible; for, though the stratagem of Hannibal was very simple in its plan and management, still he had executed it on a great scale, and had brought out the whole Roman army. There were, it is said, about forty thousand that crossed the river, and about an equal number in the Carthaginian army to oppose them. Such a body of combatants covered, of course, a large extent of ground, and the conflict that ensued was one of the most horrible scenes of the many that Hannibal assisted in enacting.

The conflict continued for many hours, the Romans getting more and more into confusion all the time. The elephants of the Carthaginians, that is, the few that now remained, made great havoc in their ranks, and finally, after a combat of some hours, the whole army was broken up and fled, some portions in compact bodies, as their officers could keep them together, and others in hopeless and inextricable confusion. They made their way back to the river, which they reached at various points up and down the stream. In the mean time, the continued rain had swollen the waters still more, the low lands were overflowed, the deep places concealed, and the broad expanse of water in the center of the stream whirled in boiling and turbid eddies, whose surface was roughened by the December breeze, and dotted every where with the drops of rain still falling.

When the Roman army was thoroughly broken up and scattered, the Carthaginians gave up the further prosecution of the contest. They were too wet, cold, and exhausted themselves to feel any ardor in the pursuit of their enemies. Vast numbers of the Romans, however, attempted to recross the river, and were swept down and destroyed by the merciless flood, whose force they had not strength enough remaining to Other portions of the troops lay hid in lurking-places to which they had retreated, until night came on, and then they made rafts on which they contrived to float themselves back across the stream. Hannibal's troops were too wet, and cold, and exhausted to go out again into the storm, and so they were unmolested in these attempts. Notwithstanding this, however, great numbers of them were carried down the stream and lost.

It was now December, too late for Hannibal to attempt to advance much further that season, and yet the way before him was open to the Apennines, by the defeat of Sempronius, for neither he nor Scipio could now hope to make another stand against him till they should receive new re-enforcements from Rome. During the winter months Hannibal had various battles and adventures, sometimes with portions and detachments of the Roman army, and sometimes with the native tribes. He was sometimes in great difficulty for want of food for his army, until at length he bribed the governor of a castle, where a Roman granary was kept, to deliver it up to him, and after that he was well supplied.

The natives of the country were, however, not at all well disposed toward him, and in the course of the winter they attempted to impede his operations, and to harass his army by every means in their power. Finding his situation uncomfortable, he moved on toward the south, and at length determined that, inclement as the season was, he would cross the Apennines.

By looking at the map of Italy, it will be seen that the great valley of the Po extends across the whole north of Italy. The valley of the Arno and of the Umbro lies south of it, separated from it by a part of the Apennine chain. This southern valley was Etruria. Hannibal decided to attempt to pass over the mountains into Etruria. He thought he should find there a warmer climate, and inhabitants more well-disposed toward him, besides being so much nearer Rome.

But, though Hannibal conquered the Alps, the Apennines conquered him. A very violent storm arose just as he reached the most exposed place among the mountains. It was intensely cold, and the wind blew the hail and snow directly into the faces of the troops, so that it was impossible for them to proceed. They halted and turned their backs to the storm, but the wind increased more and more, and was attended with terrific thunder and lightning, which filled the soldiers with alarm, as they were at such an altitude as to be themselves enveloped in the clouds from which the peals and flashes were emitted. Unwilling to retreat, Hannibal ordered the army to encamp on the spot, in the best shelter they could find. They attempted, accordingly, to pitch their tents, but it was impossible to secure them. The wind increased to a hurricane. The tent poles were unmanageable, and the canvas was carried away from its fastenings, and sometimes split or blown into rags by its flapping in the wind. The poor elephants, that is, all that were left of them from previous battles and exposures, sunk down under this intense cold and died. One only remained alive.

Hannibal ordered a retreat, and the army went back into the valley of the Po. But Hannibal was ill at ease here. The natives of the country were very weary of his presence. His army consumed their food, ravaged their country, and destroyed all their peace and happiness. Hannibal suspected them of a design to poison him or assassinate him in some other way. He was continually watching and taking precautions against these attempts. He had a great many different dresses made to be used as disguises, and false hair of different colors and fashion, so that he could alter his appearance at pleasure. This was to prevent any spy or assassin who might come into his camp from identifying him by any description of his dress and appearance. Still, notwithstanding these precautions, he was ill at ease, and at the very earliest practicable period in the spring he made a new attempt to cross the mountains, and was now successful.

On descending the southern declivities of the Apennines he learned that a new Roman army, under a new consul, was advancing toward him from the south. He was eager to meet this force, and was preparing to press forward at once by the nearest way. He found, however, that this would lead him across the lower part of the valley of the Arno, which was here very broad, and, though usually passable, was now overflowed in consequence of the swelling of the waters of the river by the melting of the snows upon the mountains. The whole country was now, in fact, a vast expanse of marshes and fens.

Still, Hannibal concluded to cross it, and, in the attempt, he involved his army in difficulties and dangers as great, almost, as he had encountered upon the Alps. The waters were rising continually; they filled all the channels and spread over extended plains. They were so turbid, too, that every thing beneath the surface was concealed, and the soldiers wading in them were continually sinking into deep and sudden channels and into bogs of mire, where many were lost. They were all exhausted and worn out by the wet and cold, and the long continuance of their exposure to it. They were four days and three nights in this situation, as their progress was, of course, extremely slow. The men, during all this time, had scarcely any sleep, and in some places the only way by which they could get any repose was to lay their arms and their baggage in the standing water, so as to build, by this means, a sort of couch or platform on which they could lie. Hannibal himself was sick too. He was attacked with a violent inflammation of the eyes, and the sight of one of them was in the end destroyed. He was not, however, so much exposed as the other officers; for there was one elephant left of all those that had commenced the march in Spain, and Hannibal rode this elephant during the four days' march through the water. There were guides and attendants to precede him, for the purpose of finding a safe and practicable road, and by their aid, with the help of the animal's sagacity, he got safely through.