Although holding a hegemony over Greece won in the Persian Wars, the Spartan-led Peloponnesian League, by making repeated demands and diplomatic affronts to Athens, made apparent its growing fear of her power.
Citing a strategy of denying any future Persian invasion a base from which to operate, Sparta had urged Athens to refrain from rebuilding her walls, as well as other Greek cities. However, suspecting a Spartan ploy and having already begun the work of construction, Athens employed subterfuge to delay the wheels of diplomacy until she could finish them.
In 464 BC, suffering another Helot rebellion and failing to make progress in the siege against their stronghold Ithome, Sparta had asked for Athens' aid along with its other allies. But after a "considerable force" arrived from Athens under the command of Cimon, Sparta, fearing the "unorthodox" politics of Athens and the possibility of its' supporting the enslaved Helots rather than fighting them, sent the Athenian contingent home while keeping on the rest of her allies.
Deeply offended by these insults and increasingly willing to support discord within the Peloponnesian League, Athens took Megara into its protection during its border dispute with the Spartan-allied Corinth, leading to open war with Corinth but not Sparta herself.
Athens, already contemptuous by Spartan treatment and now suspecting her of negotiating with factions within the city to undermine democracy and prevent the construction of the Long Walls, maneuvered to cut off the Spartan army isolated in Boeotia.
Facing either transport through waters controlled by the Athenian navy or a difficult march through the Geraneia mountain passes held by Athenian soldiers supported from Megara, the Spartans decided to wait either for the opening of a safe route home or an outright Athenian assault.
Meeting the Spartans at Tanagra, Athens fielded "their whole army, supported by 1,000 troops from Argos and by contingents from their other allies, making up altogether a force of 14,000 men." Although both sides sustained "great losses," the Spartans were victorious and returned home through the newly opened Isthmus