The Achaean League was a confederation of Greek city states in Achaea, a territory on the northern coast of the Peloponnese. An initial confederation existed during the 5th through the 4th century BC.
The Achaean League was reformed early in the 3rd century BC, and soon expanded beyond its Achaean heartland. It was first joined by the city of Sicyon, which provided it with its first great leader, Aratus of Sicyon. The League soon grew to control much of the Peloponnesus, considerably weakening the Macedonian hold on the area, but soon it ran into difficulties with the revived Sparta of Cleomenes III. Aratus was forced to call in the aid of the Macedonian King, Antigonus Doson, to defeat Cleomenes, and Antigonus re-established Macedonian control over much of the region.