Probably the best starting point is Athens loses at Marathon, leading to Persia sacking/otherwise impoverishing Attica, and then Sparta shortly afterwards as the other states are too wary to ally together without proof of victory.
Thebes and Corinth are now in prime position to take the positions of Athens and Sparta as chief candidates for hegemony, and given the trend in the Classical era, Corinth is likely to get at least one period of dominance.