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.
He supported the Phocians during the Sacred War (355-346), moved, no doubt, largely by the hatred of Thebes which he had inherited from his father: he also led the Spartan forces in the conflicts with the Thebans and their allies which arose out of the Spartan attempt to break up the city of Megalopolis.
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).