It was fought as part of the Roman–Seleucid War, pitting forces of the Roman Republic led by the consul Manius Acilius Glabrio against a Seleucid-Aetolian army of Antiochus III the Great.

When the main bodies of the armies initially clashed at the Thermopylae pass, the Seleucids managed to hold their ground, repulsing multiple Roman assaults.

However, a small Roman force under Marcus Porcius Cato managed to outflank the Seleucids from the hillside after surprising the Aetolian garrison of Fort Callidromus.

Fearing that Antiochus would seize the entirety of Asia Minor, the independent cities Smyrna and Lampsacus decided to appeal for protection from the Roman Republic.

[4] In the early spring of 196 BC, Antiochus' troops crossed to the European side of the Hellespont and began rebuilding the strategically important city of Lysimachia.

[5] In late winter 196/195 BC, Rome's erstwhile chief enemy, Carthaginian general Hannibal, fled from Carthage to Antiochus' court in Ephesus.

[11] Antiochus then shifted his attention towards rebuilding his alliance with Philip V of Macedon, which had been shattered after the latter was decisively defeated by the Romans at the 197 BC Battle of Cynoscephalae.

Only Elis, the Boeotian League and Amynander of Athamania declared their allegiance to Antiochus, the later being promised the Macedonian throne for his brother in law Philip of Megalopolis.

[18] In the meantime, Philip V and Roman propraetor Baebius launched parallel offensives in Thessaly and Athamania, quickly erasing Seleucid gains in the region.

The Aetolian force was split into two armies of equal strength, garrisoning the cities of Hypata and Heraclea in Trachis; which blocked the roads to Aetolia and Thermopylae respectively.

The ditch and earthworks situated in front of the wall stretched to the Malian Gulf, the slopes on the hills overlooking it were relatively gradual, allowing the Seleucids to man them with projectile throwers.

[24] Despite the natural strengths of the position controlled by his adversaries, Glabrio decided to launch an assault; since he held a significant numerical advantage, commanding an army of 25,000 to 30,000 soldiers.

On the night of 23 April 191 BC, Glabrio ordered Marcus Porcius Cato's and Lucius Valerius Flaccus's 2,000-man detachments to assault the Aetolian controlled forts.

When the Seleucid garrison at Chalcis followed their emperor back to Asia Minor in May 191 BC, Euboean cities immediately welcomed the Romans as liberators.

In September 191 BC, the Roman fleet defeated the Seleucids in the Battle of Corycus, enabling it to take control of several cities including Dardanus and Sestos on the Hellespont.

[30] In May 190 BC, Antiochus invaded the Kingdom of Pergamon, ravaging the countryside, besieging its capital and forcing its king, Eumenes II, to return from Greece.