Overview
The Battle of Thermopylae took place on 24 April 191 BC. 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. The Seleucids panicked and broke ranks, leading to the destruction of their force. Antiochus managed to escape the battlefield with his cavalry, departing mainland Greece soon afterwards.
Background
Following his return from his Bactrian (210–209 BC) and Indian (206–205 BC) campaigns, the Seleucid King Antiochus III the Great forged an alliance with Philip V of Macedon, seeking to jointly conquer the territories of the Ptolemaic Kingdom. In 198 BC, Antiochus emerged victorious in the Fifth Syrian War, taking over Coele-Syria and securing his south-eastern border. He then focused his attention on Asia Minor, launching a successful campaign against coastal Ptolemaic possessions. In 196 BC, Antiochus used the opportunity of Attalus I's death to assault cities controlled by the Attalid dynasty. 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. 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. In October 196 BC, Antiochus met with a delegation of Roman diplomats in Lysimachia. The Romans demanded that Antiochus withdraw from Europe and restore the autonomous status of Greek city states in Asia Minor. Antiochus countered by claiming that he was simply rebuilding the empire of his ancestor Antiochus II Theos and criticized the Romans for meddling in the affairs of Asia Minor states, whose rights were traditionally defended by Rhodes.
In late winter 196/195 BC, Rome's erstwhile chief enemy, Carthaginian general Hannibal, fled from Carthage to Antiochus' court in Ephesus. Despite the emergence of a pro-war party led by Scipio Africanus, the Roman Senate exercised restraint.
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