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.

Thus the fourth campaign, that of 557, began. Flamininus despatched a part of the fleet against the Acarnanians, who were besieged in Leucas; in Greece proper he became by stratagem master of Thebes, the capital of Boeotia, in consequence of which the Boeotians were compelled to join at least nominally the alliance against Macedonia. Content with having thus interrupted the communication between Corinth and Chalcis, he proceeded to the north, where alone a decisive blow could be struck. The great difficulties of provisioning the army in a hostile and for the most part desolate country, which had often hampered its operations, were now to be obviated by the fleet accompanying the army along the coast and carrying after it supplies sent from Africa, Sicily, and Sardinia. The decisive blow came, however, earlier than Flamininus had hoped. Philip, impatient and confident as he was, could not endure to await the enemy on the Macedonian frontier: after assembling his army at Dium, he advanced through the pass of Tempe into Thessaly, and encountered the army of the enemy advancing to meet him in the district of Scotussa. The Macedonian and Roman armies - the latter of which had been reinforced by contingents of the Apolloniates and the Athamanes, by the Cretans sent by Nabis, and especially by a strong band of Aetolians - contained nearly equal numbers of combatants, each about 26,000 men; the Romans, however, had the superiority in cavalry. In front of Scotussa, on the plateau of the Karadagh, during a gloomy day of rain, the Roman vanguard unexpectedly encountered that of the enemy, which occupied a high and steep hill named Cynoscephalae, that lay between the two camps. Driven back into the plain, the Romans were reinforced from the camp by the light troops and the excellent corps of Aetolian cavalry, and now in turn forced the Macedonian vanguard back upon and over the height. But here the Macedonians again found support in their whole cavalry and the larger portion of their light infantry; the Romans, who had ventured forward imprudently, were pursued with great loss almost to their camp, and would have wholly taken to flight, had not the Aetolian horsemen prolonged the combat in the plain until Flamininus brought up his rapidly-arranged legions. The king yielded to the impetuous cry of his victorious troops demanding the continuance of the conflict, and hastily drew up his heavy-armed soldiers for the battle, which neither general nor soldiers had expected on that day. It was important to occupy the hill, which for the moment was quite denuded of troops. The right wing of the phalanx, led by the king in person, arrived early enough to form without trouble in battle order on the height; the left had not yet come up, when the light troops of the Macedonians, put to flight by the legions, rushed up the hill. Philip quickly pushed the crowd of fugitives past the phalanx into the middle division, and, without waiting till Nicanor had arrived on the left wing with the other half of the phalanx which followed more slowly, he ordered the right phalanx to couch their spears and to charge down the hill on the legions, and the rearranged light infantry simultaneously to turn them and fall upon them in flank. The attack of the phalanx, irresistible on so favourable ground, shattered the Roman infantry, and the left wing of the Romans was completely beaten. Nicanor on the other wing, when he saw the king give the attack, ordered the other half of the phalanx to advance in all haste; by this movement it was thrown into confusion, and while the first ranks were already rapidly following the victorious right wing down the hill, and were still more thrown into disorder by the inequality of the ground, the last files were just gaining the height. The right wing of the Romans under these circumstances soon overcame the enemy's left; the elephants alone, stationed upon this wing, annihilated the broken Macedonian ranks. While a fearful slaughter was taking place at this point, a resolute Roman officer collected twenty companies, and with these threw himself on the victorious Macedonian wing, which had advanced so far in pursuit of the Roman left that the Roman right came to be in its rear. Against an attack from behind the phalanx was defenceless, and this movement ended the battle. From the complete breaking up of the two phalanxes we may well believe that the Macedonian loss amounted to 13,000, partly prisoners, partly fallen - but chiefly the latter, because the Roman soldiers were not acquainted with the Macedonian sign of surrender, the raising of the sarissae. The loss of the victors was slight. Philip escaped to Larissa, and, after burning all his papers that nobody might be compromised, evacuated Thessaly and returned home.

Simultaneously with this great defeat, the Macedonians suffered other discomfitures at all the points which they still occupied; in Caria the Rhodian mercenaries defeated the Macedonian corps stationed there and compelled it to shut itself up in Stratonicea; the Corinthian garrison was defeated by Nicostratus and his Achaeans with severe loss, and Leucas in Acarnania was taken by assault after a heroic resistance. Philip was completely vanquished; his last allies, the Acarnanians, yielded on the news of the battle of Cynoscephalae.

It was completely in the power of the Romans to dictate peace; they used their power without abusing it. The empire of Alexander might be annihilated; at a conference of the allies this desire was expressly put forward by the Aetolians. But what else would this mean, than to demolish the rampart protecting Hellenic culture from the Thracians and Celts? Already during the war just ended the flourishing Lysimachia on the Thracian Chersonese had been totally destroyed by the Thracians - a serious warning for the future. Flamininus, who had clearly perceived the bitter animosities subsisting among the Greek states, could never consent that the great Roman power should be the executioner for the grudges of the Aetolian confederacy, even if his Hellenic sympathies had not been as much won by the polished and chivalrous king as his Roman national feeling was offended by the boastings of the Aetolians, the "victors of Cynoscephalae", as they called themselves. He replied to the Aetolians that it was not the custom of Rome to annihilate the vanquished, and that, besides, they were their own masters and were at liberty to put an end to Macedonia, if they could. The king was treated with all possible deference, and, on his declaring himself ready now to entertain the demands formerly made, an armistice for a considerable term was agreed to by Flamininus in return for the payment of a sum of money and the furnishing of hostages, among whom was the king's son Demetrius, - an armistice which Philip greatly needed in order to expel the Dardani out of Macedonia.