Athens enters into an alliance with King Sitalces of Odrysian Thrace in 431 BCE after Nymphodorus, an Athenian, marries Sitalces’ sister.

Nymphodorus then negotiates an agreement between Athens and Perdiccas, where Perdiccas regains Therma.

As a result of this, Athens withdraws her support for Philip, and the Thracians promise to assist Perdiccas in capturing him.

In return, Perdiccas marches on the Chalcidians, the people he had originally persuaded to revolt.

The Boeotian League has meanwhile grown into a close-knit confederacy by 431, organized in eleven districts.

Each district, comprising one or more cities, sends a general (boeotarch), several judges, and sixty counselors to a federal government; the federal council of 660 is probably divided into four panels, each in turn convening for one year.

The vote is given only to the propertied classes.

Thebes, where the council meets, dominates the league since it controls four districts and supplies the best contingent to the federal army.

The other Spartan ally seeking to involve Sparta in a private feud with an enemy is Thebes, …

…whose attack on its neighbor Plataea (an Athenian ally) in time of peace …

…will be retrospectively recognized by Sparta as an act of war guilt.

Sparta should not have condoned it, nor should it have invaded Attica (despite the fact that Athens has placed a garrison in Plataea) so long as Athens is offering arbitration, as it seems it is.

Thucydides vacillates between two events for the beginning of the war, the invasion of Plataea and the Spartan invasion of Attica.

Both occur in 431, separated by a mere eighty days.

Pericles' actions, along with the embargo on Megaran commerce, lead Sparta in spring 431 to declare war on Athens.

The Spartans, led by Archidamus, invade Attica, hoping to destroy Athens's will to fight and to encourage Athens's subjects to rebel.

Expenditure on building has been counterbalanced by annual savings from the tribute, and enough capital has been reserved, he thinks, for a long war.

Athens moves its flocks from Attica across to …

Pericles employs a strategy designed to avoid pitched land battles and rely instead on the Athenian fleet's control of the sea to assure Athenian food supplies and secure the empire on whose resources the expensive naval policy depends.

He encourages Athenians to crowd into the city behind the Long Walls, abandoning the Attican countryside to invasion by Sparta's superior army, with whom Pericles means to decline battle. (The Athenians do, however, mount cavalry raids to harass the dispersed foot soldiers of the enemy and to keep up city morale.)

…Euboea, whose economic importance is thus raised further still.

Athens devotes naval activity in 431 BCE to stripping Corinth of assets in the northwest—of Sollium, Astacus, and …

…Cephallenia, yet …