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.

Perdiccas betrays the Athenians once again and sends one thousand troops to support a Spartan assault on Acarnania in 429 but they arrive too late to help (Thucydides 2.80).

In response to this, Sitalkes of the Thracian Odrysian state invades Macedonia with the promise of support from Athens.

This support never materializes, and Perdiccas once again uses diplomacy to ensure the survival of Macedonia.

He promises the hand of his sister in marriage to the nephew of Sitalkes, who then persuades Sitalkes to leave.

After this, Perdiccas is allied to the Spartans.