Alexander's conduct caused renewed intervention; in 364 he was defeated at Cynoscephalae by the Thebans, although the victory was dearly bought by the loss of Pelopidas, who fell in the battle.
The Spartans failed to safeguard Heraclea against the Oetaeans and Thessalians, and for a short time were displaced by the Thebans (420).
He came finally to Eretria, and, with the help of the Thebans and Lygdamis of Naxos, whom he afterwards made ruler of that island, he passed over to Attica and defeated the Athenian forces at the battle of Pallenis or Pellene.
In return for this negligence the Thebans fastened a religious quarrel upon their neighbours, and secured a penal decree against them from the Amphictyonic synod (356).
The Thebans at first accepted the terms, but on the day after, realizing that they were thus balked of their pan-Boeotian ambition, withdrew and finally severed themselves from the league.