The monograph is devoted to the study of the most famous federal state of Ancient Greece during its independent development (281-221 BC). The work describes the reasons for the flourishing of federalism in the Hellenistic Peloponnese, an attempt is made to reconstruct the events associated with the territorial growth and rise of the Achaean Union in 240-230 years. BC e., considers the connection of the federal movement in southern Greece with the international situation of that time.