Achaean League (Greek: Κοινὸν τῶν Ἀχαιῶν): confederation of Greek city states, focused on but not limited to the northern Peloponnese. It was important in the third and early second centuries BCE.
In the Hellenistic Age, new confederations of Greek city states were created. Confederation itself was nothing new. In the Classical Age, poleis had been united by Sparta in the Peloponnesian League, by Athens in the Delian League, and by Thebes in the Boeotian League. What was new, was citizenship: people were citizen of both their own city state and the league. When people resettled from one polis to another, it was comparatively easy to change one's citizenship (isopoliteia).
Among the new leagues were the Nesiotic League (on the Aegean islands, protected by the Ptolemies), the Aetolian League (in western Greece), and the Achaean League (on the northern Peloponnese).
Although an Achaean League was already in existence in the fifth century BCE, probably as a cultic amphictyony of Zeus Amarius at Aegium, the Hellenistic confederation with the same name was created in c.281 BCE by four poleis in the northern Peloponnese: Dyme, Patrae, Pharae and Tritaea. Six years later, in 275, Aegium joined, soon followed by the other towns of Achaea. The meetings of the representatives were preseded by a chairman, the strategos, who was elected for two years. In these years, a similar process took place on the northern shore of the Gulf of Corinth, where the Aetolians were uniting in the increasingly powerful Aetolian League.
In 251 BCE, Aratus of Sicyon made sure that his hometown also joined the Achaean confederation and, supported with Ptolemaic money, embarked on a policy that was directed against Macedonia. In this, he was supported by the Aetolians. Aratus was to be the leading politician: between 245 and 213, he was to be strategos no less than sixteen times. In 243, he unexpectedly seized the Acrocorinth, the citadel of Corinth, one of the three “fetters of Greece” that tied Central Greece to Macedonia.
From now on, the Achaean League controlled the isthmus of Corinth. Megara and Epidauros became members in 243, strengthening the Achaean grip on the isthmus. Full of self-confidence, the Achaean and Aetolian leagues interfered in Thessaly (239). Pro-Macedonian towns like Argos, Megalopolis, and Orchomenos were now not only cut off from their main supporter, but had to witness that Macedonia was under attack in his backyard. Megalopolis joined the Achaean League in 235 and Argos followed in 229.
By now, Sparta in the south and the Aetolians in the north became worried about the increasingly powerful Achaean League. Aratus was forced to ask help from the Macedonian king Antigonus III Doson, who demanded back the Acrocorinth (223), and intervened on the Peloponnese. Having defeated the Spartan king Cleomenes III at Sellasia (222), just north of Sparta, he had regained control of the south (222).
The next conflict was a war with the Aetolians, in which the Achaeans again allied themselves to Macedonia. However, the alliance was not to last. After the battle of Cannae (216) the Macedonian king Philip V allied his kingdom to the Carthaginian general Hannibal. The Romans reciprocitated by allying themselves to Aetolia and Achaea. In the Second Macedonian War, they broke Macedonian power in the battle of Cynoscephalae and received back Corinth and the Acrocorinth (197 BCE). Led by Philopoemen, the Achaeans secured their rear by defeating Sparta, weakened by a Roman attack.
When Rome and Macedonia went to war again in 172, the Achaeans sympatrhized with king Perseus of Macedonia. After their victory at Pydna (168), the Romans dismantled Macedonia and demanded hostages from the Achaeans. (One of these was the future historian Polybius of Megalopolis.) After the Fourth Macedonian War (150-148), the Romans made quick work of the Achaean League. The Roman consul, Mummius, sacked Corinth in 146. (It had been the league's capital since 197.
The league was essentially converted into a new province, Achaea, although inscriptions prove that for religious purposes, the council of the Achaean towns still met well into the third century CE.