Athens converted the Delian League which it led against thePersian Empire into an empire of its own, and used this power tointerfere in southern Greek city-states. The last straw was when itimposed a crippling trade ban on its neighbour Megara, a member ofthe Spartan-led Peloponnesian League. Megara appealed to thePeloponnesian League, Sparta appealed to Athens to back off, Athenspersisted, and war broke out.