Peloponnesian League
Megara defected from the Spartan-dominated Peloponnesian League (c. 460 BC) to the Delian league due to border disputes with its neighbour Corinth; this defection was one of the causes of the First Peloponnesian War (460 – c. 445 BC). By the terms of the Thirty Years' Peace of 446–445 BC Megara was forced to return to the Peloponnesian League.