Confederation of ancient Greek city-states of Arcadia. Arcadian towns had been forced to ally with Sparta by 550 BC, and most Arcadians remained faithful to Sparta during the Peloponnesian War (431–404 BC). In an effort to contain Sparta, Epaminondas of Thebes founded the city-state of Megalopolis in 371–368 BC as the seat of the Arcadian League. The League united the Arcadians for a few decades until internal discord crippled their confederation.