But the fortunate issue of the last campaign had so raised the courage or the arrogance of Philip, that, after having assured himself afresh of the neutrality of the Achaeans and the fidelity of the Macedonians by the sacri fice of some strong places and of the detested admiral Heraclides, he next spring (556) assumed the offensive and advanced into the territory of the Atintanes, with a view to form a well-entrenched camp in the narrow pass, where the Aous (Viosa) winds its way between the mountains Aeropus and Asnaus. Opposite to him encamped the Roman army reinforced by new arrivals of troops, and commanded first by the consul of the previous year, Publius Villius, and then from the summer of 556 by that year's consul, Titus Quinctius Flamininus. Flamininus, a talented man just thirty years of age, belonged to the younger generation who began to lay aside the patriotism as well as the habits of their forefathers and, though not unmindful of their fatherland, were still more mindful of themselves and of Hellenism. A skilful officer and a better diplomatist, he was in many respects admirably adapted for the management of the troubled affairs of Greece. Yet it would perhaps have been better both for Rome and for Greece, if the choice had fallen on one less full of Hellenic sympathies, and if the general despatched thither had been a man, who would neither have been bribed by delicate flattery nor stung by pungent sarcasm; who would not amidst literary and artistic reminiscences have overlooked the pitiful condition of the constitutions of the Hellenic states; and who, while treating Hellas according to its deserts, would have spared the Romans the trouble of striving after unattainable ideals.
The new commander-in-chief immediately had a conference with the king, while the two armies lay face to face inactive. Philip made proposals of peace; he offered to restore all his own conquests, and to submit to an equitable arbitration regarding the damage inflicted on the Greek cities; but the negotiations broke down, when he was asked to give up ancient possessions of Macedonia and particularly Thessaly. For forty days the two armies lay in the narrow pass of the Aous; Philip would not retire, and Flamininus could not make up his mind whether he should order an assault, or leave the king alone and reattempt the expedition of the previous year. At length the Roman general was helped out of his perplexity by the treachery of some men of rank among the Epirots - who were otherwise well disposed to Macedonia - and especially of Charops. They conducted a Roman corps of 4000 infantry and 300 cavalry by mountain paths to the heights above the Macedonian camp; and, when the consul attacked the enemy's army in front, the advance of that Roman division, unexpectedly descending from the mountains commanding the position, decided the battle. Philip lost his camp and entrenchments and nearly 2000 men, and hastily retreated to the pass of Tempe, the gate of Macedonia proper. He gave up everything which he had held except the fortresses; the Thessalian towns, which he could not defend, he himself destroyed; Pherae alone closed its gates against him and thereby escaped destruction. The Epirots, induced partly by these successes of the Roman arms, partly by the judicious moderation of Flamininus, were the first to secede from the Macedonian alliance. On the first accounts of the Roman victory the Athamanes and Aetolians immediately invaded Thessaly, and the Romans soon followed; the open country was easily overrun, but the strong towns, which were friendly to Macedonia and received support from Philip, fell only after a brave resistance or withstood even the superior foe - especially Atrax on the left bank of the Peneius, where the phalanx stood in the breach as a substitute for the wall. Except these Thessalian fortresses and the territory of the faithful Acarnanians, all northern Greece was thus in the hands of the coalition.
The south, on the other hand, was still in the main retained under the power of Macedonia by the fortresses of Chalcis and Corinth, which maintained communication with each other through the territory of the Boeotians who were friendly to the Macedonians, and by the Achaean neutrality; and as it was too late to advance into Macedonia this year, Flamininus resolved to direct his land army and fleet in the first place against Corinth and the Achaeans. The fleet, which had again been joined by the Rhodian and Pergamene ships, had hitherto been employed in the capture and pillage of two of the smaller towns in Euboea, Eretria and Carystus; both however, as well as Oreus, were thereafter abandoned, and reoccupied by Philocles the Macedonian commandant of Chalcis. The united fleet proceeded thence to Cenchreae, the eastern port of Corinth, to threaten that strong fortress. On the other side Flamininus advanced into Phocis and occupied the country, in which Elatea alone sustained a somewhat protracted siege: this district, and Anticyra in particular on the Corinthian gulf, were chosen as winter quarters. The Achaeans, who thus saw on the one hand the Roman legions approaching and on the other the Roman fleet already on their own coast, abandoned their morally honourable, but politically untenable, neutrality. After the deputies from the towns most closely attached to Macedonia - Dyme, Megalopolis, and Argos - had left the diet, it resolved to join the coalition against Philip. Cycliades and other leaders of the Macedonian party went into exile; the troops of the Achaeans immediately united with the Roman fleet and hastened to invest Corinth by land, which city - the stronghold of Philip against the Achaeans - had been guaranteed to them on the part of Rome in return for their joining the coalition. Not only, however, did the Macedonian garrison, which was 1300 strong and consisted chiefly of Italian deserters, defend with determination the almost impregnable city, but Philocles also arrived from Chalcis with a division of 1500 men, which not only relieved Corinth but also invaded the territory of the Achaeans and, in concert with the citizens who were favourable to Macedonia, wrested from them Argos. But the recompense of such devotedness was, that the king delivered over the faithful Argives to the reign of terror of Nabis of Sparta. Philip hoped, after the accession of the Achaeans to the Roman coalition, to gain over Nabis who had hitherto been the ally of the Romans; for his chief reason for joining the Roman alliance had been that he was opposed to the Achaeans and since 550 was even at open war with them. But the affairs of Philip were in too desperate a condition for any one to feel satisfaction in joining his side now. Nabis indeed accepted Argos from Philip, but he betrayed the traitor and remained in alliance with Flamininus, who, in his perplexity at being now allied with two powers that were at war with each other, had in the meantime arranged an armistice of four months between the Spartans and Achaeans.
Thus winter came on; and Philip once more availed himself of it to obtain if possible an equitable peace. At a conference held at Nicaea on the Maliac gulf the king appeared in person, and endeavoured to come to an understanding with Flamininus. With haughty politeness he repelled the forward insolence of the petty chiefs, and by marked deference to the Romans, as the only antagonists on an equality with him, he sought to obtain from them tolerable terms. Flamininus was sufficiently refined to feel himself flattered by the urbanity of the vanquished prince towards himself and his arrogance towards the allies, whom the Roman as well as the king had learned to despise; but his powers were not ample enough to meet the king's wishes. He granted him a two months' armistice in return for the evacuation of Phocis and Locris, and referred him, as to the main matter, to his government. The Roman senate had long been at one in the opinion that Macedonia must give up all her possessions abroad; accordingly, when the ambassadors of Philip appeared in Rome, they were simply asked whether they had full powers to renounce all Greece and in particular Corinth, Chalcis, and Demetrias, and when they said that they had not, the negotiations were immediately broken off, and it was resolved that the war should be prosecuted with vigour. With the help of the tribunes of the people, the senate succeeded in preventing a change in the chief command - which had often proved so injurious - and in prolonging the command of Flamininus; he obtained considerable reinforcements, and the two former commanders-in-chief, Publius Galba and Publius Villius, were instructed to place themselves at his disposal. Philip resolved once more to risk a pitched battle. To secure Greece, where all the states except the Acarnanians and Boeotians were now in arms against him, the garrison of Corinth was augmented to 6000 men, while he himself, straining the last energies of exhausted Macedonia and enrolling children and old men in the ranks of the phalanx, brought into the field an army of about 26,000 men, of whom 16,000 were Macedonian phalangitae.