The Achaean League (Greek: Κοινὸν τῶν Ἀχαιῶν, Koinon ton Akhaion "League of Achaeans") was a Hellenistic-era confederation of Greek city states on the northern and central Peloponnese. The league was named after the region of Achaea in the northwestern Peloponnese, which formed its original core. The first league was formed in the fifth century BC. The second Achaean League was established in 280 BC. As a rival of Antigonid Macedon and an ally of Rome, the league played a major role in the expansion of the Roman Republic into Greece. This process eventually led to the League's conquest and dissolution by the Romans in 146 BC.
The League represents the most successful attempt by the Greek city states to develop a form of federalism, which balanced the need for collective action with the desire for local autonomy. Through the writings of the Achaean statesman Polybius, this structure has had an influence on the constitution of the United States and other modern federal states.
The first Achaean League became active in the fifth century in the northwestern Peloponnese. After the catastrophic destruction of the ancient capital Helike by an earthquake and tsunami in 373 BC, it appears to have lapsed. Xenophon's Hellenica reports that Achaea underwent a democratic revolution in 367 and the previous ruling class was exiled.
The regional Achaean League was reformed in 281/0 BC by the communities of Dyme, Patrae, Pharae and Tritaea, joined in 275 by Aegium, which controlled the important sanctuary of Zeus Homarios. The league grew quickly to include the entire Achaean heartland, and after a decade it had ten or eleven members. The key moment for the League's transformation into a major power came in 251, when Aratus, the exiled son of a former magistrate of Sicyon, overthrew the tyranny in his native city and brought it into the Achaean League. Since the Sicyonians were of Dorian and Ionian origin, their inclusion opened the League for other national elements. Aratus, then only twenty years old, rapidly became the leading politician of the League.