Achaean Confederacy, federal organization developed by the twelve Achaean cities (see achaea) united in the cult of Zeus Hamarios. First mentioned in 453 bce as Athenian allies, Achaea's independence was guaranteed in 446 (Thirty Years Peace). In the Peloponnesian War neutrality proved impossible and Achaea fell into Sparta's sphere of influence. Common citizenship existed by 389, when it had already been extended to non-Achaean Calydon. In the 4th cent. coins were issued. Polybius (2. 41. 4–6) claimed the ‘democratic’ constitution of his own time for the early confederacy, but since in 367 the ruling class was exiled and democracy installed (Xen. Hell. 7. 1. 43) this cannot be accurate, unless the two sources mean different things by ‘democracy’. The confederacy was dissolved sometime before its revival in 281/280. It then exploited the political vacuum in Greece after the collapse of the empire of Demetrius (4) Poliorcetes, soon expanded beyond Achaea, and under the leadership of Aratus (2) of Sicyon developed a locally expansionist anti-Macedonian policy in the 240s and 230s. The admission of Arcadian cities, especially Megalopolis (235), provoked Spartan hostility and led to reconciliation with Macedonia in 224. In the ‘Cleomenic War’ (see cleomenes (2) iii), Achaea joined Antigonus (3) Doson's Hellenic League which defeated Sparta at Sellasia (222).

The confederacy also received Macedonian aid against Aetolia (see aetolian confederacy), Messene and Elis (‘Social Wars (2)’: 219–217) and subsequently remained loyal to Macedonia during the First Macedonian War (215–205). Only under severe military pressure did Achaea join Rome against Macedonia in 198 and it benefited from the Roman peace after 196, eventually receiving the unusual distinction of a treaty of alliance (perhaps 191). During this period the Achaean Confederacy under the influence and leadership of Philopoemen, Aristaenus, and Diophanes expanded to absorb the whole Peloponnese including the old enemies Sparta, Elis, and Messene; it supported Rome against Antiochus (3) III and Perseus (2) but internal pressures resulting from the sudden expansion led also to severe political tension with Rome and the internment of 1,000 leading Achaeans (including Polybius (1)) in Italy after 167. Despite the dominance of Callicrates (2) internal tensions continued to provoke Rome and culminated in the ‘Achaean War’ (146/5), which with the destruction of Corinth and partial dismemberment of the confederacy effectively ended Achaean political independence.

Polybius reports that in his time (2nd cent. bce) the whole Peloponnese, united within the Achaean Confederacy, used the same laws, weights and measures, and coinage, elected common officers, and had a common council and judges (2. 37. 10–11). The annually elected leader was a stratēgos (before 255 two), re-electable only every second year, alongside whom stood a hipparch (cavalry commander), a secretary and 10 dāmiourgoi. Council membership was probably restricted to those over 30; its size is unknown, but it doubtless represented all member-cities. The council probably met together with a popular assembly consisting of all men of military age (ekklēsia) four times a year (synodoi); only in special sessions of both bodies (synclētoi), which met as required, could questions of war, alliance, or (in the 2nd cent.) communications from the Roman senate be treated.