Point of Divergence
The POD of this timeline is the Battle of Mantinea. At this battle, Sparta was on the brink of destruction. If the Athenians had been successful, Sparta's internal government would have crumbled, the city of Sparta itself rendered harmless, and the alliance system centered around Sparta would have been sundered. Instead, through the Athenians and their allies failing to capitalize on their successes, and due to the Spartan army disobeying its commander, the battle was won by the Spartans.
In this timeline, the Spartan army follows the orders it was given, and subsequently loses the battle. With the city of Tegea captured, the Spartan Hegemony collapses.