Question

why did sparta and its allies form the peloponnesian league

Answer 1

The origin of the Peloponnesian League is sought in the sixth century BCE, when in many places in the Greek world long-lasting military coalitions were concluded. The alliance that was called "the Spartans and their allies" was one of them. It is not clear why and how it was created, but it is not a wild guess that Spartan aggression was a factor. The city-state that had conquered Messenia and reduced its inhabitants to serfdom (they became helots), needed to create a system of political control to guarantee that the Messenians would not receive outside support.