The conqueror of Zama had been selected at Rome to conduct the war on the Asiatic continent; he practically exercised the supreme command for the nominal commander-in-chief, his brother Lucius Scipio, whose intellect was insignificant, and who had no military capacity. The reserve hitherto stationed in Lower Italy was destined for Greece, the army of Glabrio for Asia: when it became known who was to command it, 5000 veterans from the Hannibalic war voluntarily enrolled, to fight once more under their beloved leader. In the Roman July, but according to the true time in March, the Scipios arrived at the army to commence the Asiatic campaign; but they were disagreeably surprised to find themselves instead involved, in the first instance, in an endless struggle with the desperate Aetolians. The senate, finding that Flamininus pushed his boundless consideration for the Hellenes too far, had left the Aetolians to choose between paying an utterly exorbitant war contribution and unconditional surrender, and thus had driven them anew to arms; none could tell when this warfare among mountains and strongholds would come to an end. Scipio got rid of the inconvenient obstacle by concerting a six-months' armistice, and then entered on his march to Asia. As the one fleet of the enemy was only blockaded in the Aegean Sea, and the other, which was coming up from the south, might daily arrive there in spite of the squadron charged to intercept it, it seemed advisable to take the land route through Macedonia and Thrace and to cross the Hellespont. In that direction no real obstacles were to be anticipated; for Philip of Macedonia might be entirely depended on, Prusias king of Bithynia was in alliance with the Romans, and the Roman fleet could easily establish itself in the straits. The long and weary march along the coast of Macedonia and Thrace was accomplished without material loss; Philip made provision on the one hand for supplying their wants, on the other for their friendly reception by the Thracian barbarians. They had lost so much time however, partly with the Aetolians, partly on the march, that the army only reached the Thracian Chersonese about the time of the battle of Myonnesus. But the marvellous good fortune of Scipio now in Asia, as formerly in Spain and Africa, cleared his path of all difficulties.

On the news of the battle at Myonnesus Antiochus so completely lost his judgment, that in Europe he caused the strongly-garrisoned and well-provisioned fortress of Lysimachia to be evacuated by the garrison and by the inhabitants who were faithfully devoted to the restorer of their city, and withal even forgot to withdraw in like manner the garrisons or to destroy the rich magazines at Aenus and Maronea; and on the Asiatic coast he opposed not the slightest resistance to the landing of the Romans, but on the contrary, while it was taking place, spent his time at Sardes in upbraiding destiny. It is scarcely doubtful that, had he but provided for the defence of Lysimachia down to the no longer distant close of the summer, and moved forward his great army to the Hellespont, Scipio would have been compelled to take up winter quarters on the European shore, in a position far from being, in a military or political point of view, secure.

While the Romans, after disembarking on the Asiatic shore, paused for some days to refresh themselves and to await their leader who was detained behind by religious duties, ambassadors from the great-king arrived in their camp to negotiate for peace. Antiochus offered half the expenses of the war, and the cession of his European possessions as well as of all the Greek cities in Asia Minor that had gone over to Rome; but Scipio demanded the whole costs of the war and the surrender of all Asia Minor. The former terms, he declared, might have been accepted, had the army still been before Lysimachia, or even on the European side of the Hellespont; but they did not suffice now, when the steed felt the bit and knew its rider. The attempts of the great-king to purchase peace from his antagonist after the Oriental manner by sums of money - he offered the half of his year's revenues! - failed as they deserved; the proud burgess, in return for the gratuitous restoration of his son who had fallen a captive, rewarded the great-king with the friendly advice to make peace on any terms. This was not in reality necessary: had the king possessed the resolution to prolong the war and to draw the enemy after him by retreating into the interior, a favourable issue was still by no means impossible. But Antiochus, irritated by the presumably intentional arrogance of his antagonist, and too indolent for any persevering and consistent warfare, hastened with the utmost eagerness to expose his unwieldy, but unequal, and undisciplined mass of an army to the shock of the Roman legions.

In the valley of the Hermus, near Magnesia at the foot of Mount Sipylus not far from Smyrna, the Roman troops fell in with the enemy late in the autumn of 564. The force of Antiochus numbered close on 80,000 men, of whom 12,000 were cavalry; the Romans - who had along with them about 5000 Achaeans, Pergamenes, and Macedonian volunteers - had not nearly half that number, but they were so sure of victory, that they did not even wait for the recovery of their general who had remained behind sick at Elaea; Gnaeus Domitius took the command in his stead. Antiochus, in order to be able even to place his immense mass of troops, formed two divisions. In the first were placed the mass of the light troops, the peltasts, bowmen, slingers, the mounted archers of Mysians, Dahae, and Elymaeans, the Arabs on their dromedaries, and the scythe-chariots. In the second division the heavy cavalry (the Cataphractae, a sort of cuirassiers) were stationed on the flanks; next to these, in the intermediate division, the Gallic and Cappadocian infantry; and in the very centre the phalanx armed after the Macedonian fashion, 16,000 strong, the flower of the army, which, however, had not room in the narrow space and had to be drawn up in double files 32 deep. In the space between the two divisions were placed 54 elephants, distributed between the bands of the phalanx and of the heavy cavalry. The Romans stationed but a few squadrons on the left wing, where the river gave protection; the mass of the cavalry and all the light armed were placed on the right, which was led by Eumenes; the legions stood in the centre. Eumenes began the battle by despatching his archers and slingers against the scythe-chariots with orders to shoot at the teams; in a short time not only were these thrown into disorder, but the camel-riders stationed next to them were also carried away, and even in the second division the left wing of heavy cavalry placed behind fell into confusion. Eumenes now threw himself with all the Roman cavalry, numbering 3000 horse, on the mercenary infantry, which was placed in the second division between the phalanx and the left wing of heavy cavalry, and, when these gave way, the cuirassiers who had already fallen into disorder also fled. The phalanx, which had just allowed the light troops to pass through and was preparing to advance against the Roman legions, was hampered by the attack of the cavalry in flank, and compelled to stand still and to form front on both sides - a movement which the depth of its disposition favoured. Had the heavy Asiatic cavalry been at hand, the battle might have been restored; but the left wing was shattered, and the right, led by Antiochus in person, had driven before it the little division of Roman cavalry opposed to it, and had reached the Roman camp, which was with great difficulty defended from its attack. In this way the cavalry were at the decisive moment absent from the scene of action. The Romans were careful not to assail the phalanx with their legions, but sent against it the archers and slingers, not one of whose missiles failed to take effect on the densely-crowded mass. The phalanx nevertheless retired slowly and in good order, till the elephants stationed in the interstices became frightened and broke the ranks. Then the whole army dispersed in tumultuous flight; an attempt to hold the camp failed, and only increased the number of the dead and the prisoners. The estimate of the loss of Antiochus at 50,000 men is, considering the infinite confusion, not incredible; the legions of the Romans had never been engaged, and the victory, which gave them a third continent, cost them 24 horsemen and 300 foot soldiers. Asia Minor submitted; including even Ephesus, whence the admiral had hastily to withdraw his fleet, and Sardes the residence of the court.