The Achaean League (Ancient Greek: Κοινὸν τῶν Ἀχαιῶν, romanized: Koinon ton Akhaion, lit. '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. Although the first Achaean League is much less well documented than its later revival, it maintained a recognizable federal structure through the early Hellenistic period, but later fell into a period of dormancy under growing Macedonian influence. The more famous second Achaean League was established in 280 BC. First it was an ally of Antigonid Macedon at the Cleomenean War, Social War and First Macedonian War and a rival of the Aetolian League and Sparta. As a rival of Antigonid Macedon and an ally of the Roman Republic since the Second Macedonian War, the league played a major role in the expansion of Rome 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 historian and statesman Polybius, this structure has had an influence on the constitution of the United States and other modern federal states.
