The battle of Thermopylae of 191 B.C. ended the Greek phase of the war between Rome and the Seleucid emperor Antiochus III. Antiochus had crossed into Greece from Asia Minor at the head of small army, hoping to find allies amongst the Greeks. He had been disappointed in this expectation – only the Aetolian League, who had invited him into Greece in the first place, offered him troops, and even then not as many as he had hoped.
The Romans responded by sending an army to Greece, commanded by the consul M. Acilius Glabrio. He was more successful in finding allies, most notably gaining the support of Philip V of Macedonia, who only a few years before had been crushingly defeated by the Romans at Cynoscephalae (Second Macedonian War). Between them Philip and the Romans quickly recaptured all of Antiochus’ conquests in Thessaly.
Antiochus decided to defend the pass of Thermopylae, where the greater Roman numbers would not be so telling. This position allowed him to remain in communication with Aetolia, and protected the crucial naval base at Chalcis. Antiochus defended the pass himself, with his 10,500 men, posting his slingers on the heights above the pass and his phalanx behind strong earthworks. The Aetolians were given the task of guarding his left flank, leaving 2,000 men at Heraclea in Trachis and posting 2,000 men in the forts that guarded the Asopus gorge and the mountain tracks that the Persians had used.
Unfortunately for Antiochus the Romans had read the history books. They may have had as many as 40,000 men, and so on the night before the Roman attack they could afford to send 2,000 men around his western flank. On the day of the battle the Romans began with a frontal assault on his position. The first attack failed under a hail of missile weapons from the heights, and even when a second attack broke through the first Seleucid line, they were held off by Antiochus’ dug-in phalanx.
The turning point of the battle came when the Roman flanking force appeared behind Antiochus’ position, and defeated the Aetolian troops guarding the col of Callidromus. The Seleucid army in the pass broke and fled, suffering heavy losses in the retreat. Antiochus was only able to rally 500 men at Elatea. He then retreated to Chalcis, before setting sail for Ephesus and Asia Minor.
The war in Greece continued across the summer of 191, and saw Philip V recover some of the areas he had lost to the Aetolians after the Second Macedonian War. The Aetolians were then given permission to appear to the Senate, effectively suing for peace. At the same time the Romans turned their attention to an invasion of Asia Minor, winning a major naval battle at Corycus before winter ended the campaign of 191.