The sixth book of 'Greek' begins with a reference to the invasion of the Thebans into Phocis and the response of the Lacedaemonians to the request of the Phocians for help with the sending of King Cleombrotus.

A digression by the author follows about the events in Thessaly, as Polydamas from Pharsalus informs the Spartans about the great power that is emerging with the strong man Jason, who becomes the tyrant of the Pharai with leadership plans for all of Greece.

Returning to the narration related to the Athens-Sparta front, Xenophon speaks about the peace between them and the recall of Timotheus, but also the immediate violation of it with the occupation of Corcyra by the Lacedaemonians and their involvement in new conflicts with the Athenian admiral Iphicrates.

Meanwhile, the undertaking of dynamic activities by Thebes against the allies of the Athenians leads to a rapprochement between Athens and Sparta with the sending of an embassy and their participation in a conference for the peace of all Greek cities.

At this conference, the Thebans insist on signing as representatives of the Boeotians (and not just Thebes), and for this reason, they are declared outlaws. Then the Lacedaemonians invade Boeotia with Cleombrotus to force the Thebans to obey the terms of peace, but in the battle that took place at Leuctra in 371 BC, Sparta faces an unexpected defeat, where Cleombrotus is killed.

New forces of Lacedaemonians and allies are sent against the Thebans led by Agisilaus' son Archidamus, the Athenians refuse to help the Thebans, and through the mediation of the Thessalian leader Jason, peace is signed between the Thebans and the Lacedaemonians.

With a new digression, Xenophon then refers to the inglorious end of Jason in Thessaly. A new conference follows in Athens to adhere to the terms of the 'king's peace', but the disagreements of the cities of the Peloponnese, the initiatives of the Mantineans against the decisions of Sparta, etc., lead to the invasion of Epaminondas, the leader of the Thebans, into the Peloponnese for assistance to the Arcadians and their joint invasion into Laconia.

These developments cause the rapprochement of Athens and Sparta and the sending of aid from the Athenians to the Lacedaemonians with Iphicrates, but due to winter, both the Athenians and the Thebans ultimately withdraw from the Peloponnese. In the author's view, Iphicrates could have been more effective against the Thebans who had invaded the Peloponnese.