By 378, Sparta was trying unsuccessfully to take over the entire harbor of Athens, Piraeus. For this, Athens conclude an alliance with Thebes and create the Second Athenian Union. The battles between the parties took place with varying success and by 378 Sparta, led by Clembrot, invaded Boeotia. She makes a flanking march from Megara in order to reach the direction of Thebes. On transported to them, the Theban army moved under the command of Epaminondas. The battle took place on a plain with an area of just over 2 km. The Spartan army numbered about a thousand horsemen and ten thousand hoplites. The Theban army had 6 thousand hoplites and a thousand horsemen. On the left flank of the Spartan army were the Peloponnesian allies, and the right flank was occupied by the main army of Sparta. The cavalry was located in the center of the army.
Epaminondas used military cunning and took advantage of the "oblique" battle formation. His left flank was occupied by all his main forces, the depth of which was 50 ranks. At the rear of his column, he placed a "sacred detachment" consisting of 300 selected Thebans.
The Theban army was located on a hill, while its weak right flank rested against the hill. The army of Sparta is located proudly along the entire valley. With the first attack, the Theban cavalry defeated the enemy's cavalry, after which the line attacked the right flank of the Spartans, breaking through their entire formation. The Spartans suffered a heavy defeat.
After this victory, Epaminondas invaded the Peloponnese. But Athens did not like it and they went over to the side of Sparta. And with the help of the Afiski commander Iphikrates, the Theban invasion was avoided.
By 362, Epaminondas had made his way across the Isthmus of Corinth again. A major battle took place at the city of Mantinea. On the side of Sparta, the Athenian wars fought again, and the formation was deeper than in the previous battle. The "oblique" formation of the Thebans was again able to cut through the enemy's ranks, but the Spartan troops reorganized in time and
stop the onslaught of the enemy. At this moment, the general Epaminondas was unfortunately mortally wounded. The battle was not ended with the victory of one of the parties, but still the death of the commander undermined the morale of the troops and they retreated to the Isthmus of Corinth.
After this war, none of the states could claim complete domination in all of Greece. By 357 Athens went to war against its allies and by 355 the naval alliance had disintegrated.