At first the war turned chiefly towards Campania. Hannibal appeared in good time to protect its capital, which he prevented from being invested; but he was unable either to wrest any of the Campanian towns held by the Romans from their strong Roman garrisons, or to prevent - in addition to a number of less important country towns - Casilinum, which secured his passage over the Volturnus, from being taken by the two consular armies after an obstinate defence. An attempt of Hannibal to gain Tarentum, with the view especially of acquiring a safe landing-place for the Macedonian army, proved unsuccessful. Meanwhile the Bruttian army of the Carthaginians under Hanno had various encounters in Lucania with the Roman army of Apulia; here Tiberius Gracchus sustained the struggle with good results, and after a successful combat not far from Beneventum, in which the slave legions pressed into service had distinguished themselves, he bestowed liberty and burgess-rights on his slave-soldiers in the name of the people.
In the following year (541) the Romans recovered the rich and important Arpi, whose citizens, after the Roman soldiers had stolen into the town, made common cause with them against the Carthaginian garrison. In general the bonds of the symmachy formed by Hannibal were relaxing; a number of the leading Capuans and several of the Bruttian towns passed over to Rome; even a Spanish division of the Phoenician army, when informed by Spanish emissaries of the course of events in their native land, passed from the Carthaginian into the Roman service.
The year 542 was more unfavourable for the Romans in consequence of fresh political and military errors, of which Hannibal did not fail to take advantage. The connections which Hannibal maintained in the towns of Magna Graecia had led to no serious result; save that the hostages from Tarentum and Thurii, who were kept at Rome, were induced by his emissaries to make a foolhardy attempt at escape, in which they were speedily recaptured by the Roman posts. But the injudicious spirit of revenge displayed by the Romans was of more service to Hannibal than his intrigues; the execution of all the hostages who had sought to escape deprived them of a valuable pledge, and the exasperated Greeks thenceforth meditated how they might open their gates to Hannibal. Tarentum was actually occupied by the Carthaginians in consequence of an understanding with the citizens and of the negligence of the Roman commandant; with difficulty the Roman garrison maintained itself in the citadel. The example of Tarentum was followed by Heraclea, Thurii, and Metapontum, from which town the garrison had to be withdrawn in order to save the Tarentine Acropolis. These successes so greatly increased the risk of a Macedonian landing, that Rome felt herself compelled to direct renewed attention and renewed exertions to the Greek war, which had been almost totally neglected; and fortunately the capture of Syracuse and the favourable state of the Spanish war enabled her to do so.
At the chief seat of war, in Campania, the struggle went on with very varying success. The legions posted in the neighbourhood of Capua had not yet strictly invested the city, but had so greatly hindered the cultivation of the soil and the ingathering of the harvest, that the populous city was in urgent need of supplies from without. Hannibal accordingly collected a considerable supply of grain, and directed the Campanians to receive it at Beneventum; but their tardiness gave the consuls Quintus Flaccus and Appius Claudius time to come up, to inflict a severe defeat on Hanno who protected the grain, and to seize his camp and all his stores. The two consuls then invested the town, while Tiberius Gracchus stationed himself on the Appian Way to prevent Hannibal from approaching to relieve it But that brave officer fell in consequence of the shameful stratagem of a perfidious Lucanian; and his death was equivalent to a complete defeat, for his army, consisting mostly of those slaves whom he had manumitted, dispersed after the fall of their beloved leader. So Hannibal found the road to Capua open, and by his unexpected appearance compelled the two consuls to raise the blockade which they had barely begun. Their cavalry had already, before Hannibal's arrival, been thoroughly defeated by the Phoenician cavalry, which lay as a garrison in Capua under Hanno and Bostar, and by the equally excellent Campanian horse. The total destruction of the regular troops and free bands in Lucania led by Marcus Centenius, a man imprudently promoted from a subaltern to be a general, and the not much less complete defeat of the negligent and arrogant praetor Gnaeus Fulvius Flaccus in Apulia, closed the long series of the misfortunes of this year. But the stubborn perseverance of the Romans again neutralized the rapid success of Hannibal, at least at the most decisive point. As soon as Hannibal turned his back on Capua to proceed to Apulia, the Roman armies once more gathered around that city, one at Puteoli and Volturnum under Appius Claudius, another at Casilinum under Quintus Fulvius, and a third on the Nolan road under the praetor Gaius Claudius Nero. The three camps, well entrenched and connected with one another by fortified lines, precluded all access to the place, and the large, inadequately provisioned city could not but find itself compelled by the mere investment to surrender at no distant time, should no relief arrive. As the winter of 542-3 drew to an end, the provisions were almost exhausted, and urgent messengers, who were barely able to steal through the well-guarded Roman lines, requested speedy help from Hannibal, who was at Tarentum, occupied with the siege of the citadel. With 33 elephants and his best troops he departed by forced marches from Tarentum for Campania, captured the Roman post at Caiatia, and took up his camp on Mount Tifata close by Capua, in the confident expectation that the Roman generals would, now raise the siege as they had done the year before. But the Romans, who had had time to entrench their camps and their lines like a fortress, did not stir, and looked on unmoved from their ramparts, while on one side the Campanian horsemen, on the other the Numidian squadrons, dashed against their lines. A serious assault could not be thought of by Hannibal; he could foresee that his advance would soon draw the other Roman armies after him to Campania, if even before their arrival the scarcity of supplies in a region so systematically foraged did not drive him away. Nothing could be done in that quarter.
Hannibal tried a further expedient, the last which occurred to his inventive genius, to save the important city. After giving the Campanians information of his intention and exhorting them to hold out, he started with the relieving army from Capua and took the road for Rome. With the same dexterous boldness which he had shown in his first Italian campaigns, he threw himself with a weak army between the armies and fortresses of the enemy, and led his troops through Samnium and along the Valerian Way past Tibur to the bridge over the Anio, which he passed and encamped on the opposite bank, five miles from the city. The children's children of the Romans still shuddered, when they were told of "Hannibal at the gate"; real danger there was none. The country houses and fields in the neighbourhood of the city were laid waste by the enemy; the two legions in the city, who went forth against them, prevented the investment of the walls. Besides, Hannibal had never expected to surprise Rome by a coup de main, such as Scipio soon afterwards executed against New Carthage, and still less had he meditated a siege in earnest; his only hope was that in the first alarm part of the besieging army of Capua would march to Rome and thus give him an opportunity of breaking up the blockade. Accordingly after a brief stay he departed. The Romans saw in his withdrawal a miraculous intervention of the gods, who by portents and visions had compelled the wicked man to depart, when in truth the Roman legions were unable to compel him; at the spot where Hannibal had approached nearest to the city, at the second milestone on the Appian Way in front of the Capene gate, with grateful credulity the Romans erected an altar to the god "who turned back and protected" (Rediculus Tutanus), Hannibal in reality retreated, because this was part of his plan, and directed his march towards Capua. But the Roman generals had not committed the mistake on which their opponent had reckoned; the legions remained unmoved in the lines round Capua, and only a weak corps had been detached on the news of Hannibal's march towards Rome. When Hannibal learned this, he suddenly turned against the consul Publius Galba, who had imprudently followed him from Rome, and with whom he had hitherto avoided an engagement, vanquished him, and took his camp by storm.