The king sued for peace and consented to the terms proposed by the Romans, which, as usual, were just the same as those offered before the battle and consequently included the cession of Asia Minor. Till they were ratified, the army remained in Asia Minor at the expense of the king; which came to cost him not less than 3000 talents (730,000 pounds). Antiochus himself in his careless fashion soon consoled himself for the loss of half his kingdom; it was in keeping with his character, that he declared himself grateful to the Romans for saving him the trouble of governing too large an empire. But with the day of Magnesia Asia was erased from the list of great states; and never perhaps did a great power fall so rapidly, so thoroughly, and so ignominiously as the kingdom of the Seleucidae under this Antiochus the Great. He himself was soon afterwards (567) slain by the indignant inhabitants of Elymais at the head of the Persian gulf, on occasion of pillaging the temple of Bel, with the treasures of which he had sought to replenish his empty coffers.

The Roman government, after having achieved the victory, had to arrange the affairs of Asia Minor and of Greece. If the Roman rule was here to be erected on a firm foundation, it was by no means enough that Antiochus should have renounced the supremacy in the west of Asia Minor. The circumstances of the political situation there have been set forth above[6]. The Greek free cities on the Ionian and Aeolian coast, as well as the kingdom of Pergamus of a substantially similar nature, were certainly the natural pillars of the new Roman supreme power, which here too came forward essentially as protector of the Hellenes kindred in race. But the dynasts in the interior of Asia Minor and on the north coast of the Black Sea had hardly yielded for long any serious obedience to the kings of Asia, and the treaty with Antiochus alone gave to the Romans no power over the interior. It was indispensable to draw a certain line within which the Roman influence was henceforth to exercise control. Here the element of chief importance was the relation of the Asiatic Hellenes to the Celts who had been for a century settled there. These had formally apportioned among them the regions of Asia Minor, and each one of the three cantons raised its fixed tribute from the territory laid under contribution. Doubtless the burgesses of Pergamus, under the vigorous guidance of their presidents who had thereby become hereditary princes, had rid themselves of the unworthy yoke; and the fair afterbloom of Hellenic art, which had recently emerged afresh from the soil, had grown out of these last Hellenic wars sustained by a national public spirit. But it was a vigorous counterblow, not a decisive success; again and again the Pergamenes had to defend with arms their urban peace against the raids of the wild hordes from the eastern mountains, and the great majority of the other Greek cities probably remained in their old state of dependence[7].

If the protectorate of Rome over the Hellenes was to be in Asia more than a name, an end had to be put to this tributary obligation of their new clients; and, as the Roman policy at this time declined, much more even in Asia than on the Graeco-Macedonian peninsula, the possession of the country on its own behalf and the permanent occupation therewith connected, there was no course in fact left but to carry the arms of Rome up to the limit which was to be staked off for the domain of Rome's power, and effectively to inaugurate the new supremacy among the inhabitants of Asia Minor generally, and above all in the Celtic cantons.

This was done by the new Roman commander-in-chief, Gnaeus Manlius Volso, who relieved Lucius Scipio in Asia Minor. He was subjected to severe reproach on this score; the men in the senate who were averse to the new turn of policy failed to see either the aim, or the pretext, for such a war. There is no warrant for the former objection, as directed against this movement in particular; it was on the contrary, after the Roman state had once interfered in Hellenic affairs as it had done, a necessary consequence of this policy. Whether it was the right course for Rome to undertake the protectorate over the Hellenes collectively, may certainly be called in question; but regarded from the point of view which Flamininus and the majority led by him had now taken up, the overthrow of the Galatians was in fact a duty of prudence as well as of honour. Better founded was the objection that there was not at the time a proper ground of war against them; for they had not been, strictly speaking, in alliance with Antiochus, but had only according to their wont allowed him to levy hired troops in their country. But on the other side there fell the decisive consideration, that the sending of a Roman military force to Asia could only be demanded of the Roman burgesses under circumstances altogether extraordinary, and, if once such an expedition was necessary, everything told in favour of carrying it out at once and with the victorious army that was now stationed in Asia. So, doubtless under the influence of Flamininus and of those who shared his views in the senate, the campaign into the interior of Asia Minor was undertaken in the spring of 565. The consul started from Ephesus, levied contributions from the towns and princes on the upper Maeander and in Pamphylia without measure, and then turned northwards against the Celts. Their western canton, the Tolistoagii, had retired with their belongings to Mount Olympus, and the middle canton, the Tectosages, to Mount Magaba, in the hope that they would be able there to defend themselves till the winter should compel the strangers to withdraw. But the missiles of the Roman slingers and archers - which so often turned the scale against the Celts unacquainted with such weapons, almost as in more recent times firearms have turned the scale against savage tribes - forced the heights, and the Celts succumbed in a battle, such as had often its parallels before and after on the Po and on the Seine, but here appears as singular as the whole phenomenon of this northern race emerging amidst the Greek and Phrygian nations. The number of the slain was at both places enormous, and still greater that of the captives. The survivors escaped over the Halys to the third Celtic canton of the Trocmi, which the consul did not attack. That river was the limit at which the leaders of Roman policy at that time had resolved to halt. Phrygia, Bithynia, and Paphlagonia were to become dependent on Rome; the regions lying farther to the east were left to themselves.

The affairs of Asia Minor were regulated partly by the peace with Antiochus (565), partly by the ordinances of a Roman commission presided over by the consul Volso. Antiochus had to furnish hostages, one of whom was his younger son of the same name, and to pay a war-contribution - proportional in amount to the treasures of Asia - of 15,000 Euboic talents (3,600,000 pounds), a fifth of which was to be paid at once, and the remainder in twelve yearly instalments. He was called, moreover, to cede all the lands which he possessed in Europe and, in Asia Minor, all his possessions and claims of right to the north of the range of the Taurus and to the west of the mouth of the Cestrus between Aspendus and Perga in Pamphylia, so that he retained nothing in Asia Minor but eastern Pamphylia and Cilicia. His protectorate over its kingdoms and principalities of course ceased. Asia, or, as the kingdom of the Seleucids was thenceforth usually and more appropriately named, Syria, lost the right of waging aggressive wars against the western states, and in the event of a defensive war, of acquiring territory from them on the conclusion of peace; lost, moreover, the right of navigating the sea to the west of the mouth of the Calycadnus in Cilicia with vessels of war, except for the conveyance of envoys, hostages, or tribute; was further prevented from keeping more than ten decked vessels in all, except in the case of a defensive war, from taming war-elephants, and lastly from the levying of mercenaries in the western states, or receiving political refugees and deserters from them at court. The war vessels which he possessed beyond the prescribed number, the elephants, and the political refugees who had sought shelter with him, he delivered up. By way of compensation the great-king received the title of a friend of the Roman commonwealth. The state of Syria was thus by land and sea completely and for ever dislodged from the west; it is a significant indication of the feeble and loose organization of the kingdom of the Seleucidae, that it alone, of all the great states conquered by Rome never after the first conquest desired a second appeal to the decision of arms.