The immediate effect, indeed, of this union of the Italian and Sicilian Greeks under one control was a closer concert also on the part of their antagonists. Carthage and Rome now converted their old commercial treaties into an offensive and defensive league against Pyrrhus (475), the tenor of which was that, if Pyrrhus invaded Roman or Carthaginian territory, the party which was not attacked should furnish that which was assailed with a contingent on its own territory and should itself defray the expense of the auxiliary troops; that in such an event Carthage should be bound to furnish transports and to assist the Romans also with a war fleet, but the crews of that fleet should not be obliged to fight for the Romans by land; that lastly, both states should pledge themselves not to conclude a separate peace with Pyrrhus. The object of the Romans in entering into the treaty was to render possible an attack on Tarentum and to cut off Pyrrhus from his own country, neither of which ends could be attained without the co-operation of the Punic fleet; the object of the Carthaginians was to detain the king in Italy, so that they might be able without molestation to carry into effect their designs on Syracuse[5]. It was accordingly the interest of both powers in the first instance to secure the sea between Italy and Sicily. A powerful Carthaginian fleet of 120 sail under the admiral Mago proceeded from Ostia, whither Mago seems to have gone to conclude the treaty, to the Sicilian straits. The Mamertines, who anticipated righteous punishment for their outrage upon the Greek population of Messana in the event of Pyrrhus becoming ruler of Sicily and Italy, attached themselves closely to the Romans and Carthaginians, and secured for them the Sicilian side of the straits. The allies would willingly have brought Rhegium also on the opposite coast under their power; but Rome could not possibly pardon the Campanian garrison, and an attempt of the combined Romans and Carthaginians to gain the city by force of arms miscarried. The Carthaginian fleet sailed thence for Syracuse and blockaded the city by sea, while at the same time a strong Phoenician army began the siege by land (476). It was high time that Pyrrhus should appear at Syracuse: but, in fact, matters in Italy were by no means in such a condition that he and his troops could be dispensed with there. The two consuls of 476, Gaius Fabricius Luscinus, and Quintus Aemilius Papus, both experienced generals, had begun the new campaign with vigour, and although the Romans had hitherto sustained nothing but defeat in this war, it was not they but the victors that were weary of it and longed for peace. Pyrrhus made another attempt to obtain accommodation on tolerable terms. The consul Fabricius had handed over to the king a wretch, who had proposed to poison him on condition of being well paid for it. Not only did the king in token of gratitude release all his Roman prisoners without ransom, but he felt himself so moved by the generosity of his brave opponents that he offered, by way of personal recompense, a singularly fair and favourable peace. Cineas appears to have gone once more to Rome, and Carthage seems to have been seriously apprehensive that Rome might come to terms. But the senate remained firm, and repeated its former answer. Unless the king was willing to allow Syracuse to fall into the hands of the Carthaginians and to have his grand scheme thereby disconcerted, no other course remained than to abandon his Italian allies and to confine himself for the time being to the occupation of the most important seaports, particularly Tarentum and Locri. In vain the Lucanians and Samnites conjured him not to desert them; in vain the Tarentines summoned him either to comply with his duty as their general or to give them back their city. The king met their complaints and reproaches with the consolatory assurance that better times were coming, or with abrupt dismissal. Milo remained behind in Tarentum; Alexander, the king's son, in Locri; and Pyrrhus, with his main force, embarked in the spring of 476 at Tarentum for Syracuse.
By the departure of Pyrrhus the hands of the Romans were set free in Italy; none ventured to oppose them in the open field, and their antagonists everywhere confined themselves to their fastnesses or their forests. The struggle however was not terminated so rapidly as might have been expected; partly in consequence of its nature as a warfare of mountain skirmishes and sieges, partly also, doubtless, from the exhaustion of the Romans, whose fearful losses are indicated by a decrease of 17,000 in the burgess-roll from 473 to 479. In 476 the consul Gaius Fabricius succeeded in inducing the considerable Tarentine settlement of Heraclea to enter into a separate peace, which was granted to it on the most favourable terms. In the campaign of 477 a desultory warfare was carried on in Samnium, where an attack thoughtlessly made on some entrenched heights cost the Romans many lives, and thereafter in southern Italy, where the Lucanians and Bruttians were defeated. On the other hand Milo, issuing from Tarentum, anticipated the Romans in their attempt to surprise Croton: whereupon the Epirot garrison made even a successful sortie against the besieging army. At length, however, the consul succeeded by a stratagem in inducing it to march forth, and in possessing himself of the undefended town (477). An incident of more moment was the slaughter of the Epirot garrison by the Locrians, who had formerly surrendered the Roman garrison to the king, and now atoned for one act of treachery by another. By that step the whole south coast came into the hands of the Romans, with the exception of Rhegium and Tarentum. These successes, however, advanced the main object but little. Lower Italy itself had long been defenceless; but Pyrrhus was not subdued so long as Tarentum remained in his hands and thus rendered it possible for him to renew the war at his pleasure, and the Romans could not think of undertaking the siege of that city. Even apart from the fact that in siege-warfare, which had been revolutionized by Philip of Macedonia and Demetrius Poliorcetes, the Romans were at a very decided disadvantage when matched against an experienced and resolute Greek commandant, a strong fleet was needed for such an enterprise, and, although the Carthaginian treaty promised to the Romans support by sea, the affairs of Carthage herself in Sicily were by no means in such a condition as to enable her to grant that support.
The landing of Pyrrhus on the island, which, in spite of the Carthaginian fleet, had taken place without interruption, had changed at once the aspect of matters there. He had immediately relieved Syracuse, had in a short time united under his sway all the free Greek cities, and at the head of the Sicilian confederation had wrested from the Carthaginians nearly their whole possessions. It was with difficulty that the Carthaginians could, by the help of their fleet which at that time ruled the Mediterranean without a rival, maintain themselves in Lilybaeum; it was with difficulty, and amidst constant assaults, that the Mamertines held their ground in Messana. Under such circumstances, agreeably to the treaty of 475, it would have been the duty of Rome to lend her aid to the Carthaginians in Sicily, far rather than that of Carthage to help the Romans with her fleet to conquer Tarentum; but on the side of neither ally was there much inclination to secure or to extend the power of the other. Carthage had only offered help to the Romans when the real danger was past; they in their turn had done nothing to prevent the departure of the king from Italy and the fall of the Carthaginian power in Sicily. Indeed, in open violation of the treaties Carthage had even proposed to the king a separate peace, offering, in return for the undisturbed possession of Lilybaeum, to give up all claim to her other Sicilian possessions and even to place at the disposal of the king money and ships of war, of course with a view to his crossing to Italy and renewing the war against Rome. It was evident, however, that with the possession of Lilybaeum and the departure of the king the position of the Carthaginians in the island would be nearly the same as it had been before the landing of Pyrrhus; the Greek cities if left to themselves were powerless, and the lost territory would be easily regained. So Pyrrhus rejected the doubly perfidious proposal, and proceeded to build for himself a war fleet. Mere ignorance and shortsightedness in after times censured this step; but it was really as necessary as it was, with the resources of the island, easy of accomplishment. Apart from the consideration that the master of Ambracia, Tarentum, and Syracuse could not dispense with a naval force, he needed a fleet to conquer Lilybaeum, to protect Tarentum, and to attack Carthage at home as Agathocles, Regulus, and Scipio did before or afterwards so successfully. Pyrrhus never was so near to the attainment of his aim as in the summer of 478, when he saw Carthage humbled before him, commanded Sicily, and retained a firm footing in Italy by the possession of Tarentum, and when the newly-created fleet, which was to connect, to secure, and to augment these successes, lay ready for sea in the harbour of Syracuse.