Battle of Leuctra
Sparta's interference and invasion of Thebes in 382 BC gave the latter a very good reason to join the league. However her behaviour within the league became difficult and Athens started to realise that Thebes was not necessarily to be trusted. For example, Thebes destroyed Plataea in 372 BC, which had only recently been refounded. Athens started to think about negotiating peace with Sparta; it was while Athens was discussing this with Sparta that Thebes defeated the Spartan army conclusively at Battle of Leuctra (371 BC).