The Achaean League was a confederation of 10-12 city-states in the northern Peloponnesus. Like the Aetolian League, it had a mixed constitution run by delegates from the member communities, and held elections every year. However, it had no headquarters and for each session met in a different place. Otherwise it was more centralized; the president was the League's commander in chief, though he could not serve for two consecutive terms. In fact, around 190 the League members gave up their separate laws and coinages, making them a single state without a capital.
Its importance grew in the 4th century, when we find it fighting in the Theban wars (368-362 B.C.), against Philip (338) and Antipater (330). About 288 Antigonos Gonatas dissolved the league, which had furnished a useful base for pretenders against Cassander's regency; but by 280 four towns combined again, and before long the ten surviving cities' of Achaea had renewed their federation. Antigonos' preoccupation during the Celtic invasions, Sparta's prostration after the Chremonidean campaigns, the wealth amassed by Achaean adventurers abroad and the subsidies of Egypt, the standing foe of Macedonia, all enhanced the league's importance .The Achaean League's most important leader was Aratus of Sicyon, who was president nearly every other year from 245 until his death in 213. Under Aratus the League saw its greatest successes, taking Corinth from Macedonia in 243, adding Megalopolis (in Arcadia) eight years later, and acquiring Argos in 229. Like Pericles, he was a persuasive speaker whose objective at first was to free the Peloponnesus from Macedonian domination, and he is credited with bringing into the confederation many of the principal cities of Greece. But he was none the less instrumental for the subsequent Macedonian domination of the Peloponnesus, for while fighting Cleomenes III of Sparta and the Aetolian League he changed his policy towards Macedonia and called in Antigonus III when Sparta looked liked winning. He successfully oversaw the reduction of Sparta, so the successors of Aratus alternated between resisting Macedonia and resisting Rome. Aratus probably also organized the new federal constitution, the character of which, owing to the scanty and somewhat perplexing nature of our evidence, we can only approximately determine. The league embraced an indefinite number of city-states which maintained their internal independence practically undiminished, and through their several magistrates, assemblies and law-courts exercised all traditional powers of self-government. Only in matters of foreign politics and war was their competence restricted. The most important of the latter presidents, Philopoemon, incorporated Sparta into the League in 192, followed by Elis and Messenia. This gave the League control over nearly all of the Peloponnesus. When the Romans finished their conquest of Greece in 146, they disbanded the Achaean League, but gave its name to the new province they created there..
The central government, like that of the constituent cities, was of a democratic cast. The chief legislative powers resided in a popular assembly in which every member of the league over thirty years of age could speak and vote. This body met for three days in spring and autumn at Aegium to discuss the league's policy and elect the federal magistrates. Whatever the number of its attendant burgesses, each city counted but one on a division. Extraordinary assemblies could be convoked at any time or place on special emergencies. A council of 120 unpaid delegates, selected from the local councils, served partly as a committee for preparing the assembly's programme, partly as an administrative board which received embassies, arbitrated between contending cities and exercised penal jurisdiction over offenders against the constitution. But perhaps some of these duties concerned the dicastae and gerousia, whose functions are nowhere described. The chief magistracy was the strategic, (tenable every second year), which combined with an unrestricted command in the field a large measure of civil authority. Besides being authorized to veto motions, the strategos (general) had practically the sole power of introducing measures before the assembly. The ten elective demimgi, who presided over this body, formed a kind of cabinet, and perhaps acted as departmental chiefs. We also hear of an under-strategos, a secretary, a cavalry commander and an admiral. All these higher officers were unpaid. Philopoemen (q.v.) transferred the seat of assembly from town to town by rotation, and placed dependent communities on an equal footing with their former suzerains.
The league prescribed uniform laws, standards and coinage; it summoned contingents, imposed taxes and fined or coerced refractory members.
The first federal wars were directed against Macedonia; in 266-263 the league fought in the Chremonidean league, in 243-241 against Antigonos Gonatas and Aetolia, between 239 and 229 with Aetolia against Demetrius. A greater danger arose (227-223) from the attacks of Cleomenes III. Owing to Aratus's irresolute generalship, the indolence of the rich citizens and the inadequate provision for levying troops and paying mercenaries, the league lost several battles and much of its territory; but rather than compromise with the Cleomenes the assembly negotiated with Antigonos Doson, who recovered the lost districts but retained Corinth for himself (223-221). Similarly the Achaeans could not check the incursions of Aetolian adventurers in 220-218, and when Philip V. came to the rescue he made them tributary and annexed much of the Peloponnese. Under Philopoemen the league with a reorganized army defeated the Aetolians (210) and Spartans (207, 201). After their benevolent neutrality during the Macedonian war the Roman general, T. Quinctius Flamininus, restored all their lost possessions and sanctioned the incorporation of Sparta and Messenia (191), thus bringing the entire Peloponnese under Achaean control. The league even sent troops to Pergamum against Antiochus (190). The annexation of Aetolia and Zacynthus was forbidden by Rome. Moreover, Sparta and Messenia always remained unwilling members. After Philopoemen's death the aristocrats initiated a strongly phito-Roman policy, declared war against King Perseus and denounced all sympathizers with Macedonia. This agitation induced the Romans to deport 1,000 prominent Achaeans, and, failing proof of treason against Rome, to detain them seventeen years. These hostages, when restored in 150, swelled the ranks of the proletariat opposition, whose leaders, to cover their maladministration at home, precipitated a war by attacking Sparta in defiance of Rome. The federal troops were routed in central Greece by Q. Caecilius Metellus Macedonicus, and again near Corinth by L. Mummius Achaicus (146). The Romans now dissolved the league (in effect, if not in name), and took measures to isolate the communities (see POLYBIUS). Augustus instituted an Achaean synod comprising the dependent cities of Peloponnese and central Greece; this body sat at Argos and acted as guardian of Hellenic sentiment.