The battle of Cithaeron (376 BC) was a minor Spartan defeat that prevented them from conducting a fourth invasion of Boeotia in four years (Theban-Spartan War).
Sparta had seized control of Thebes in 382 BC, but the pro-Spartan government had been expelled in 379. The Spartans responded with a series of invasions of Boeotia. In 379 King Cleombrotus got close to Thebes, but withdrew without achieving anything. In 378 and 377 King Agesilaus II led the invasions, and managed to get into Boeotia and again got close to Thebes, but both invasions ended without achieving much. In the aftermath of the 377 campaign Agesilaus burst a vein in his leg, and was unfit for the campaign of 376.
This meant that King Cleombrotus took command for a second time. In 377 the Spartan garrison of Thespiae had been ordered to keep the passes across the Cithaeron range open, to allow the Spartans to move from Attica into Boeotia, but in 376 the passes were defended by Theban and Athenian troops. It is possible that the Spartan garrisons at Plataea and Tanagra, on the Boeotian side of the mountains, had been defeated before this, thus explaining their inability to help the invasion.
Cleombrotus ordered his peltasts to seize the pass. As the Spartans approached, the defenders sortied, and killed forty of the attackers. Cleombrotus wasn't as persistent as Agesilaus, and he decided that this made the entire invasion impossible. He retreated, disbanded the army, and returned home.
Cleombrotus's poor performance in Boeotia in 379 and 376 might have played a part in the Spartan disaster at Leuctra in 371 BC, where Cleombrotus was defeated and killed, triggering the start of a dramatic decline in Spartan power. The king may have been motivated to fight by a desire to restore his reputation.