I lack knowledge about the Republican Rome and its wars of expansion. However, here is a wonderful (albeit simplistic) overview of the Roman Battle of Thermopylae (plate number 1).
Rome's Battle of Thermophylae was part of a larger conflict between the Roman and Seleucid Empires to control the region. The Roman forces, led by consul Manius Acilius Glabrio, defeated the Seleucid-Aetolian army under Antiochus III the Great at the historic site where the Spartans had been famously defeated by the Persians many years before in 480 BCE.
The Seleucid forces were defeated at Thermopylae and then suffered another devastating defeat by the Romans at the Battle of Magnesia. The Battle of Magnesia, along with the earlier Roman victory against Philip of Macedon at the Battle of Cynoscepahlae in 197 BC, signified the end of the perceived dominance of the Macedonian phalanx on the battlefields of the ancient world.
https://en.m.wikipedia.org/wiki/Battle_of_Thermopylae_(191_BC)#:~:text=The Battle of Thermopylae took,of Antiochus III the Great.&text=View of the Thermopylae pass from the area of the Phocian Wall.