The battle of Tegyra (Spring 375 BC) saw an outnumbered Theben defeat a force of Spartan hoplites twice its own size, an early sign that the Thebans were no longer intimidated by the impressive reputation of the Spartans (Theban-Spartan War, 379-371 BC).
The war had been triggered by the Spartan seizure of Thebes in 382 and the overthrow of the pro-Spartan government and Spartan garrison in 379. A series of Spartan attempts to invade Boeotia in 379, 378, 377 and 376 had all ended in failure, and in 376 the Spartans hadn't even reached Boeotia. This gave the Thebans a change to eliminate most of the Spartan garrisons and allies in Boeotia, and by 375 only Plataea and Orchomenus were outside the revived Boeotian League.
In 375 the Spartans had two morai, about 1,000 men, under the command of Gorgoleon and Theopompus, at Orchomenus in western Boeotia, on the north-western shore of Lake Copais. In the spring of 375 the Spartans decided to use these troops to raid Eastern Locris, the area to the north of Orchomenus. Pelopidas, one of the Theban Liberators, and by 375 the commander of the Theban Sacred Band, decided to try and attack Orchomenus while the Spartans were away. This gave him 300 elite hoplites, and he also took 200 cavalry, so the assumption is that he was expected some sort of local support within Orchomenus. The Thebans advanced around the northern side of the lake, the side nearest to Eastern Locris. As they approached Orchomenus from the east, the Thebans discovered that the Spartans had moved a fresh garrison into the city, and Pelopidas decided to retreat.
At the same time as the Thebans were moving east, the Spartan morai were moving south from Eastern Locris, on their way back to Orchomenus. The two armies ran into each other at Tegyra, on the northern shores of Lake Copais, where there was enough space to fight a hoplite battle. Some of the Thebans panicked, and one is reported to have run up to Pelopidas saying 'we have fallen into our enemy's hands'. He replied 'why not say that they have fallen into ours', indicating his intention to attack.
Pelopidas's plan was to punch a hole through the Spartan line. He ordered his cavalry to harass the Spartan hoplites, while the Sacred Band was ordered to form into close order, possibly into a deep column. The two sides then charged, and a fierce struggle began. Fairly early in the battle the two Spartan polemarchs, Gorgoleon and Theopompus, were killed, and Spartan morale began to waver. During the battle of Coronea (394 BC) the Thebans had taken a chance to escape once a hole had been opened in the Spartan line, and this may have been in the Spartan's minds now. A gap opened up in the middle of the Spartan line, and the Thebans had their escape route. However the Spartans had underestimated Pelopidas's determination. Instead of taking the chance to escape, he used the gap to attack the flank and rear of the Spartan formation. Hoplite formations didn’t hold up well to this sort of attack, and the Spartan force was soon in full flight. The Theban cavalry pursued for a short distance, but Pelopidas was worried that the garrison from Orchomenus might have been pursuing his force, and didn’t press his advantage.
The Thebans built a victory trophy, and then continued on their way back to Thebes. Although the battle of Tegyra wasn't a large scale encounter, it was significant as the first time a force of Spartan hoplites had been defeated by a smaller force of similar troops (earlier defeats had been at the hands of much larger forces, as at Thermopylae). The same year also saw the Spartans suffer a naval defeat at Alyzeia, ending an attempt to ship troops across the Corinthian Gulf. By now all sides were ready for peace, and later in the year a peace treaty was agreed, that at least temporarily ended the fighting.