418 BCE (POD): In response to the capture of Episaurus by the Athenians, Argives, Achaeans, and Eleans, King Agis of Sparta amasses his army to march on the Delian League, but the ephors punish Agis' torpidity by enabling the xymbouloi (ten advisors) to consent on martial action.
Battle of Mantinea: After early allied successess, the Spartan xymbouloi quarrel with King Agis over strategic maneuvers, delaying the Spartan reaction to Argive and Athenian advances. Athenian General Laches and the Argive generals submit to their mutinously eager hoplites, and the allies assault the Spartiates. The elite Argive 'Thousand' massacre the Lacedaemonians. King Agis and the xymbouloi are killed in the engagement.
The allies hold council outside of Lacedaemon; the Argives, Eleans, and Achaeans press for the destruction of Sparta and the enslavement of the populace. General Laches' refuses to consent to an invasion of Laecadoman without approval from the demos. In Athens, the ekklesia overwhelmingly votes to provide General Alcibades with Athenian troops for a campaign into Lacedaemonia. In the aftermath of the vote, the non-Achean Arcadians defect from the Spartans, and join the Delian army, which is aggrandized by contingents from Sparta's former allies.
Supported by the the Arcadians, Athenians, Argives, Acheans, and Eleans, Alcibades crosses the Evrotas River — an inconceivable feat in Hellenistic history. The Athenians and their allies ravage Laconia, including the port of Gythium, and liberate the perioeci from the Spartan grip. Alcibades briefly returns north to block Corinthian intervention, before the allied army spins south and presses towards Messenia.
The Spartans send emissaries to Alcibades to press for peace, but Alcibades refuses negotiation until the Argive army occupies Messenia and frees the Helots . The Athenian reinforcements arrive soon thereafter, and Alcibades develops fortifications for the newly liberated Messenia. Despite the entreaties of Argos, Alcibades receives the Spartan ephors and forwards their request for peace to the ekklessia. The demos demands the liberation of the helots and the submission of Sparta to the Delian League, but refuse to consider the destruction of Sparta. The question of Sparta's constitution remains undecided, and Alcibades refuses to attack Sparta. The Argivives, distraught with Alcibades' refusal, depart from Laecadoman.
Alcibades moves camp to beyond the Evrotas River in order to coerce the Spartans into submission. The surviving Spartiates await for the decision from King Pleistoanax and the ephors, but the lengthy consideration is interrupted by an Argive attack on Sparta. Although the Spartiates rout the Argive incursion, Alcibades resolves to strike, and encircles Sparta as the Argives withdraw. In an effort to not inflict a deeper shame upon Sparta and force the Spartiates to defend Sparta, Alcibades seeks terms with Pleistoanax. The Spartan emissaries agree to submit to the terms of the Athenian populace on condition that Alcibades withdraws from Lacedaemon. Alcibades consents, ravages the Corinthian countryside, and retires to Athens for the winter. Thucydides relates the celebration that overtook Athens following Alcibades' return and the relegation of Sparta to second-tier polis.
417 BCE: General Pagondas of Thebes, confident after his previous victory at Delium, rebukes pleas from other Boeotian cities in the koinon to sue for peace. Strategoes Nicias advises the ekklesia to make peace with Thebes, while Alcibades and Hyperbolus request an embargo and invasion of Boeotia. Nicias worries that the Athenian troops will be defeated again in Boeotia, but news of a pro-democratic coup in Corinth galvanizes the demos to an aggressive policy. Corinthian ambassadors arrive in Athens and settle the war in the Peloponnese.
Alcibades launches a major offensive against Boeotia — determined to (re)capture Plataea and reduce Theban power over the Boeotian koinon. Pagondas' previous usage of deeper ranks, reserves, cavalry interventions, light-armed skirmishes (peltasts), and gradual tactical changes is deployed against him by Alcibades at the Battle of Plataea.
The Athenians send garrisons to Messenia and delegations to Sparta. The delegations catch a first-hand report of the economic catastrophe of Sparta, which has been forced to contract its premier Spartiates to Achaemenid Kings as mercanaries.