In the beginning of spring 563 the Roman staff arrived at Apollonia. The commander-in-chief was Manius Acilius Glabrio, a man of humble origin, but an able general feared both by his soldiers and by the enemy; the admiral was Gaius Livius; and among the military tribunes were Marcus Porcius Cato, the conqueror of Spain, and Lucius Valerius Flaccus, who after the old Roman wont did not disdain, although they had been consuls, to re-enter the army as simple war-tribunes. They brought with them reinforcements in ships and men, including Numidian cavalry and Libyan elephants sent by Massinissa, and the permission of the senate to accept auxiliary troops to the number of 5000 from the extra-Italian allies, so that the whole number of the Roman forces was raised to about 40,000 men. The king, who in the beginning of spring had gone to the Aetolians and had thence made an aimless expedition to Acarnania, on the news of Glabrio's landing returned to his head-quarters to begin the campaign in earnest. But incom prehensibly, through his own negligence and that of his lieutenants in Asia, reinforcements had wholly failed to reach him, so that he had nothing but the weak army - now further decimated by sickness and desertion in its dissolute winter-quarters - with which he had landed at Pteleum in the autumn of the previous year. The Aetolians too, who had professed to send such enormous numbers into the field, now, when their support was of moment, brought to their commander-in-chief no more than 4000 men. The Roman troops had already begun operations in Thessaly, where the vanguard in concert with the Macedonian army drove the garrisons of Antiochus out of the Thessalian towns and occupied the territory of the Athamanes. The consul with the main army followed; the whole force of the Romans assembled at Larisa.
Instead of returning with all speed to Asia and evacuating the field before an enemy in every respect superior, Antiochus resolved to entrench himself at Thermopylae, which he had occupied, and there to await the arrival of the great army from Asia. He himself took up a position in the chief pass, and commanded the Aetolians to occupy the mountain-path, by which Xerxes had formerly succeeded in turning the Spartans. But only half of the Aetolian contingent was pleased to comply with this order of the commander-in-chief; the other 2000 men threw themselves into the neighbouring town of Heraclea, where they took no other part in the battle than that of attempting during its progress to surprise and plunder the Roman camp. Even the Aetolians posted on the heights discharged their duty of watching with remissness and reluctance; their post on the Callidromus allowed itself to be surprised by Cato, and the Asiatic phalanx, which the consul had meanwhile assailed in front, dispersed, when the Romans hastening down the mountain fell upon its flank. As Antiochus had made no provision for any case and had not thought of retreat, the army was destroyed partly on the field of battle, partly during its flight; with difficulty a small band reached Demetrias, and the king himself escaped to Chalcis with 500 men. He embarked in haste for Ephesus; Europe was lost to him all but his possessions in Thrace, and even the fortresses could be no longer defended Chalcis surrendered to the Romans, and Demetrias to Philip, who received permission - as a compensation for the conquest of the town of Lamia in Achaia Phthiotis, which he was on the point of accomplishing and had then abandoned by orders of the consul - to make himself master of all the communities that had gone over to Antiochus in Thessaly proper, and even of the territories bordering on Aetolia, the districts of Dolopia and Aperantia. All the Greeks that had pronounced in favour of Antiochus hastened to make their peace; the Epirots humbly besought pardon for their ambiguous conduct, the Boeotians surrendered at discretion, the Eleans and Messenians, the latter after some struggle, submitted to the Achaeans. The prediction of Hannibal to the king was fulfilled, that no dependence at all could be placed upon the Greeks, who would submit to any conqueror. Even the Aetolians, when their corps shut up in Heraclea had been compelled after obstinate resistance to capitulate, attempted to make their peace with the sorely provoked Romans; but the stringent demands of the Roman consul, and a consignment of money seasonably arriving from Antiochus, emboldened them once more to break off the negotiations and to sustain for two whole months a siege in Naupactus. The town was already reduced to extremities, and its capture or capitulation could not have been long delayed, when Flamininus, constantly striving to save every Hellenic community from the worst consequences of its own folly and from the severity of his ruder colleagues, interposed and arranged in the first instance an armistice on tolerable terms. This terminated, at least for the moment, armed resistance in Greece.
A more serious war was impending in Asia - a war which appeared of a very hazardous character on account not so much of the enemy as of the great distance and the insecurity of the communications with home, while yet, owing to the short-sighted obstinacy of Antiochus, the struggle could not well be terminated otherwise than by an attack on the enemy in his own country. The first object was to secure the sea. The Roman fleet, which during the campaign in Greece was charged with the task of interrupting the communication between Greece and Asia Minor, and which had been successful about the time of the battle at Thermopylae in seizing a strong Asiatic transport fleet near Andros, was thenceforth employed in making preparations for the crossing of the Romans to Asia next year and first of all in driving the enemy's fleet out of the Aegean Sea. It lay in the harbour of Cyssus on the southern shore of the tongue of land that projects from Ionia towards Chios; thither in search of it the Roman fleet proceeded, consisting of 75 Roman, 24 Pergamene, and 6 Carthaginian, decked vessels under the command of Gaius Livius. The Syrian admiral, Polyxenidas, a Rhodian emigrant, had only 70 decked vessels to oppose to it; but, as the Roman fleet still expected the ships of Rhodes, and as Polyxenidas relied on the superior seaworthiness of his vessels, those of Tyre and Sidon in particular, he immediately accepted battle. At the outset the Asiatics succeeded in sinking one of the Carthaginian vessels; but, when they came to grapple, Roman valour prevailed, and it was owing solely to the swiftness of their rowing and sailing that the enemy lost no more than 23 ships. During the pursuit the Roman fleet was joined by 25 ships from Rhodes, and the superiority of the Romans in those waters was now doubly assured. The enemy's fleet thenceforth kept the shelter of the harbour of Ephesus, and, as it could not be induced to risk a second battle, the fleet of the Romans and allies broke up for the winter; the Roman ships of war proceeded to the harbour of Cane in the neighbourhood of Pergamus. Both parties were busy during the winter in preparing for the next campaign. The Romans sought to gain over the Greeks of Asia Minor; Smyrna, which had perseveringly resisted all the attempts of the king to get possession of the city, received the Romans with open arms, and the Roman party gained the ascendency in Samos, Chios, Erythrae, Clazomenae, Phocaea, Cyme, and elsewhere. Antiochus was resolved, if possible, to prevent the Romans from crossing to Asia, and with that view he made zealous naval preparations - employing Polyxenidas to fit out and augment the fleet stationed at Ephesus, and Hannibal to equip a new fleet in Lycia, Syria, and Phoenicia; while he further collected in Asia Minor a powerful land army from all regions of his extensive empire. Early next year (564) the Roman fleet resumed its operations. Gaius Livius left the Rhodian fleet - which had appeared in good time this year, numbering 36 sail - to observe that of the enemy in the offing of Ephesus, and went with the greater portion of the Roman and Pergamene vessels to the Hellespont in accordance with his instructions, to pave the way for the passage of the land army by the capture of the fortresses there. Sestus was already occupied and Abydus reduced to extremities, when the news of the defeat of the Rhodian fleet recalled him. The Rhodian admiral Pausistratus, lulled into security by the representations of his countryman that he wished to desert from Antiochus, had allowed himself to be surprised in the harbour of Samos; he himself fell, and all his vessels were destroyed except five Rhodian and two Coan ships; Samos, Phocaea, and Cyme on hearing the news went over to Seleucus, who held the chief command by land in those provinces for his father.