Theban expansionism is bound to drive Athens…

Helike, located in Achaea, northern Peloponnesos, two kilometers (twelve stadia) from the Corinthian Gulf and near the city of Boura, like it a member of the Achaean League, had been founded in the Bronze Age, becoming the principal city of Achaea.

The poet Homer states that the city of Eliki participated in the Trojan War with one ship.

Later, following its fall to the Achaeans, Eliki led the Achaean League, an association that joined twelve neighboring cities in an area including today's town of Aigion.

Also known as Dodekapolis (from the Greek words dodeka, meaning twelve, and polis, meaning city), Eliki became a cultural and religious center with its own coinage.

Finds from ancient Eliki are limited to two fifth century copper coins, now housed in the Staatliches Museum, Berlin.

The obverse shows the head of Poseidon, the city's patron, and the reverse his trident.

There was a temple dedicated to the Helikonian Poseidon.

Helike had founded colonies including Priene in Asia Minor and Sybaris in South Italy.

Its panhellenic temple and sanctuary of Helikonian Poseidon are known throughout the Classical world, and second only in religious importance to Delphi.

The city is destroyed in 373 BCE, two years before the Battle of Leuctra, during a winter night.

Several events are construed in retrospect as having warned of the disaster: some "immense columns of flame" appeared, and five days previously, all animals and vermin had fled the city, going toward Keryneia.

The city and a space of twelve stadia below it sink into the earth and are covered over by the sea.

All the inhabitants perish without a trace, except for a few building fragments projecting from the sea.

Ten Spartan ships anchored in the harbor are dragged down with it.

An attempt involving two thousand men to recover bodies is unsuccessful.

Aegium takes possession of its territory.

The city was thought to be legend until 2001, when it was rediscovered in the Helike delta.

Modern research attributes the catastrophe to an earthquake and accompanying tsunami, which destroyed and submerged the city.

Sparta has steadily lost ground in central Greece following the southern city-state’s expulsion from Thebes.

The Thebans energetically centralize Boeotia under their own leadership; for instance, they again gain control of the unfortunate Plataea, which must have been resettled at some point, or perhaps just gradually, after the Peloponnesian War.

…Athens.

In this respect, the Second Athenian Confederacy, with its political justification in terms of anti-Spartan sentiment, has already been superseded by events.

There are other causes for concern within the confederacy.

Tribute by another name has been levied, reasonably so, for the western operations of 373: ships cost money, and Athens does not have great reserves, as it had in the fifth century.

Perhaps more disquieting in its implications is …

…the Athenian garrison on Cephallenia (attested by an inscription of 373; there may, however, have been special factors, and it is not known how long the garrison remained).

Iphicrates had served the Persians as a mercenary commander after the Corinthian War, then returned to Athens.

His expedition in 373 to relieve Corcyra of a Spartan siege is successful.

The Persians had sacked and burned the temple of Apollo at Abae in 480 BCE, but the famous oracle continues to be consulted—e.g., by the Thebans in 371 BCE before the Battle of Leuctra.

The general war in Greece is ended at a Panhellenic peace conference in 371 BCE (which, in fact, results in another King's Peace), but Sparta and Athens combine to refuse recognition to the Theban federation by insisting that each city of Boeotia should be a separate party to the treaty, while Thebes claims that its federation should be treated as a single unit.

Epaminondas, who is boeotarch (one of the five magistrates of the federation), maintains this position, even when it leads to the exclusion of Thebes from the peace treaty.

The Spartans have an army stationed on Thebes's western frontier, waiting to follow up their diplomatic success by a crushing military attack.

After a breach in the negotiations, signaled by a rhetorical duel between Epaminondas (“a man famous for culture and philosophy,” as his fellow Boeotian Plutarch will describe him half a millennium later) and Agesilaus, who refuses to recognize him as the representative of all Boeotia, the Spartans, led by their other king, Cleombrotus, invade.

Epaminondas is ready with a tactical innovation in the Battle of Leuctra in 371 BCE, twenty days after the peace conference.

Instead of the usual advances of heavily armed infantry drawn up in an equal number of ranks over the whole front, he masses his troops on the left wing, in a further development of the Delium arrangement of 424 BCE, to the unprecedented depth of fifty ranks against an overall Spartan depth of twelve.

This provides a flexible “tail,” or reserve force on the left that can be deployed as the course of the battle suggests.

The decision about whether, when, and how to deploy it will be the general's, whose influence on the outcome of the battle is thus greater than had been usual hitherto.

By placing the best troops on the left, the Thebans aim to knock out the best Spartan troops, who are positioned opposite them, occupying the right wing in the traditional hoplite manner.

Finally, by marching forward obliquely (rather than straightforwardly, as is customary), the Thebans increase the punch administered by this deepened left.

The novelty consists in striking the enemy first at their strongest, instead of their weakest, point, with such crushing force that the attack is irresistible.

The force of the Theban advance overwhelms the Spartans.

The Spartans lose about one thousand men, four hundred of them full Spartan citizens.

The defeat, in which democrats from Argos take part, inflicts such heavy losses on the very limited numbers of the Spartan soldiers that it seriously threatens the possibility of raising another Spartan army.

The Boeotian federation is thus saved.

Thebes, in alliance with Athens, has successfully fought off Sparta and reestablished its traditional leadership in a federation of the cities of Boeotia.

Thespiae had become autonomous as stipulated in the King's Peace of 386 BCE that resolved the Corinthian War, maintaining autonomy until 373 BCE, when the Thebans had subdued the city.

Exiled from Boeotia, the Thespians had arrived in Athens along with the Plataeans seeking aid.

Still, they send a contingent to fight against the Spartans at the Battle of Leuctra in 371 BCE.

The Boeotarch Epaminondas allows the Thespians to withdraw before the battle, along with other Boeotians who nurse a grudge against Thebes.

The city is restored at some later point.