federal state or “sympolity” of Aetolia, in ancient Greece. Probably based on a looser tribal community, it was well-enough organized to conduct negotiations with Athens in 367 BC. It became by c. 340 one of the leading military powers in Greece. Having successfully resisted invasions by Macedonia in 322 and 314–311, the league rapidly grew in strength during the ensuing period of Macedonian weakness, expanding into Delphi (centre of the Amphictyonic Council) and allying with Boeotia (c. 300).

It was mainly responsible for driving out a major Gallic invasion of Greece in 279. About 270 it gained an alliance with Antigonus Gonatas, king of Macedonia, which lasted until his death (240 or 239). In 245 the league confirmed its influence in central Greece by the defeat of the Boeotians at Chaeronea. By the end of the 3rd century the league's power extended to Cephallenia and several Aegean islands; soon afterward, however, it lost ground to Macedonia.

From 239 to 229 the league joined Achaea against Demetrius II of Macedonia, but the provinces of Thessaly that they seized on the death of Demetrius were promptly recovered by his successor, Antigonus Doson. Meanwhile, eastern Phocis and Boeotia detached themselves from the confederacy. Then Aetolian raids on Achaean territory (220) led to a war with Philip V of Macedonia and many members of Antigonus Doson's Greek League. Philip expelled the Aetolians from the Peloponnese and marched into Aetolia, sacking the federal capital of Thermum. He made peace with Aetolia in 217, but in 211 and 200–197 the Aetolians fought with Rome against Philip. When their cavalry prevailed at Cynoscephalae (197), the Romans handed over Dolopia, Phocis, and Eastern Locris to the Aetolians but withheld their former Thessalian possessions. Resentful, Aetolia attempted to fight Rome (192), soliciting the support of the Seleucid king Antiochus III; but Aetolian forces failed to hold Thermopylae and brought on the defeat of Antiochus at Magnesia. The Romans refused all compromises and in 189 BC restricted the league to Aetolia proper and assumed control of its foreign relations. The importance of the league as an independent state was at an end, and by the time of Sulla its functions were purely nominal.

The federal constitution of Aetolia, probably a model for that of the Achaean League, provided for two main ruling bodies: a primary assembly, composed of all adult male citizens and presided over by the annually elected general (stratēgos (strategus)), which met at Thermum to elect officials and at various cities to transact other business; and a council (boulē or synedrion), to supervise administration, in which cities were represented in proportion to their populations. Apoklētoi, a small group of at least 30 who were assigned essential duties in wartime, assisted the stratēgos, who had complete control in the field. Leadership within the league was always kept in Aetolian hands, since the more distant states, which were linked to the confederacy by isopolity (potential citizenship), had full civil, but no political, rights.