Antiochus' landed in Greece but was forced to retreat across the Aegean after being defeated at the Battle of Thermopylae by the consul of 191 BC, Manius Acilius Glabrio.

The consul of 190 BC, Lucius Cornelius Scipio, then pursued Antiochus into Asia Minor with the support of the Pergamene king Eumenes II.

The Romans thereby gained uncontested hegemony over the Greek city-states in the Balkans and Asia Minor while also largely excluding the Seleucids from the Mediterranean.

[9] With the conquests of Antiochus and the Roman victory in the Second Punic War, the Aegean was now flanked by two great powers on its east and west.

[10] Roman influence continued expanding as a result of the Second Macedonian War (200–197 BC), fought between the republic and Philip V of Macedon.

[11] The Roman senate, influenced by a senatorial "circle of 'eastern experts'" led by Publius Sulpicius Galba Maximus, who were veterans of the First Macedonian War, sent an embassy to Philip with an ultimatum.

[12] Over the next three years, the Romans fought Philip and by 197 BC were victorious; in the aftermath, the Aegean's interstate politics had shifted considerably.

[16] The Romans, in the peace after the Macedonian war, declared four towns – formerly Philip's possessions – to be free even though they were within Antiochus' sphere of influence.

[17] The Romans also – in the aftermath of the war – proclaimed freedom for all Greeks, explicitly including even those in Asia Minor under Antiochus' control.

[19] A later embassy reached the king at Lysimachia and demanded Antiochus' withdrawal from Ptolemaic lands in Asia Minor, his withdrawal from lands formerly Philip's, and that he refrain from attacking any Greek cities (as all Greek cities had been declared free); the Romans had no right to demand the last element and Antiochus deftly brushed off Roman demands by appealing to his historic claims in the region and protesting the lack of any legitimate Roman interest in Asia Minor after his marriage alliance with Ptolemy and his own declaration of freedom for the Greek cities in Asia Minor.

After Flaminius, one of the ambassadors, spoke to the Aetolian League, it responded by passing a decree to invite Antiochus to liberate Greece and arbitrate the dispute between Rome and Aetolia.

The Romans responded by dispatching the praetor Aulus Atilius Serranus with a fleet to the Peloponnese and Marcus Baebius Tamphilus with two legions to Epirus.

Further troops were levied and, in the new year of 191 BC, placed under the command of Manius Acilius Glabrio to conduct the war "against Antiochus and those in his empire".

[32] The spring of 191 BC saw the Macedonians enter the war against the Aetolian League – they operated independently of the Romans – and occupy a number of towns in Thessaly.

[38] Prevented from crossing the Aegean directly, the Scipios stayed in Europe, where they oversaw a six-month truce with the Aetolians so that they could send envoys to the senate in Rome negotiate a peace.

[40] Late in the year, some time around the middle of December, the decisive battle of the war took place near Magnesia ad Sipylum.

The senate, with no desire to maintain a military presence in Asia Minor, gave Rhodes Lycia and Caria south of the River Maeander while Eumenes received the rest.

[57] Manlius Vulso, after defeating some Galatian Gauls in Anatolia and seizing from them (and, of political importance, not from Greeks) substantial plunder, marched to Pamphylia to receive the first large instalment of Antiochus' war indemnity.

Hearing of the arrival of the senatorial legation, he then moved to Apamea and there with them precisely defined the Taurus line, which started Cape Sarpedon and ran through the upper portions of the River Tanais.

[55] A number of disarmament provisions were also included: among other things, Antiochus would pledge to desist from the use of war elephants and be prohibited from sailing past Cape Sarpedon.

[60] The territories formerly Antiochean were re-parcelled immediately after the treaty; this was to the substantial advantage of Eumenes, who later supported Roman intervention against his enemy Macedon in the Third Macedonian War.

[63] Antiochus' immediate acceptance of terms after Magnesia reflected a prudent belief that further war between Rome and the Seleucid empire would be mutually ruinous.

Antiochus still had more men and significant space to trade for time, especially given that the Romans would be forced to besiege every city along the Royal Road down to Cilicia.

However, giving up the whole of Asia Minor created for Antiochus and his successors a massive buffer zone which allowed for a prolonged peace between the Roman and Seleucid empires.

Instead the Romans sought to undermine their adversary through diplomatic channels, threats and bribery by spurring smaller states to declare war on the Seleucids.

[66] After Antiochus' death on 3 July 187 BC, his successor Seleucus IV Philopator immediately started rebuilding his navy as funds became available, but largely did not provoke and remained aloof from the Romans.

[69] Some sources, such as Appian and Zonaras, indicate that the Romans attempted after the death of Antiochus IV to enforce the provisions of the treaty by burning Seleucid ships and hamstringing elephants.

[70] However, that the Seleucids had both ships and elephants had been known by this point for two decades and Polybius' explanation of the matter makes no reference to the treaty, instead explaining Roman action in terms of military opportunism.

[71] Rome, through the middle of the second century, took a cordial approach toward the Seleucids; a Roman diplomatic embassy arriving shortly after a military parade in Antioch made no mention of the treaty on its return to the senate even though it surely would have seen elephants.