Thermos was an ancient Greek sanctuary, which served as the regular meeting place of the Aetolian League. Its focal point was the temple of Apollo Thermios of about 630 to 610 BC, "one of the earliest developed Doric temples known and a monument of primary importance for our knowledge of the history of Greek architecture". The most famous survivals are the Archaic terracotta metopes decorated with painted scenes from mythology, which are among the earliest examples of this art form in Greece. What is left of these, and other finds from the site, are now in the museum at Thermos, with a selection of the best pieces in National Archaeological Museum, Athens.

The Aetolian League was a confederation of tribal communities and cities in ancient Greece centered in Aetolia in Central Greece. It was probably established during the early Hellenistic era, in opposition to Macedon and the Achaean League. Two annual meetings were held at Thermon and Panaetolika. The league occupied Delphi from 290 BC and steadily gained territory until, by the end of the 3rd century BC, it controlled the whole of central Greece with the exception of Attica and Boeotia. At its peak, the league's territory included Locris, Malis, Dolopes, parts of Thessaly, Phocis, and Acarnania. In the latter part of its power, certain Greek city-states joined the Aetolian League such as the Arcadian cities of Mantineia, Tegea, Phigalia and Kydonia on Crete.