A conflict with Corinth was for the free city Corcyra reason to ask Athens for help in 433. Both cities signed a defensive treaty, and Athens did indeed help Corcyra effectively when it was attacked by Corinth. This increased the feelings of unrest between Athens and Corinth, and it got even worse because of more incidents. It is not very surprising that Corinth and Megara, the two cities who were most dependent on their trade, had problems with the Athenian attempts of expansion.

Corinth on its turn asked Sparta to interfere in the conflicts between the cities. Sparta had two options. It could choose the side of Corinth and Megara, and join the war, but then there was the danger of new revolts of the Messenian helots. The past had learnt them how dangerous these revolts could be, especially now the number of Spartans was so dangerously small. On the other hand, not joining the war could mean the end of the Peloponessian League, and the start of a new anti-Sparta League under the leadership of the arch-enemy of Sparta, Argos. This also meant the end of Sparta as a dominating Polis.

Both options were not very attractive, but helping Corinth seemed the least dangerous. Sparta claimed that they entered the war as they wanted to end the Athenian imperialism and free all Greeks from whatever form of oppression. Of course was the real reason self-preservation, just like this was the reason for Pericles and the Athenians. Nevertheless became the always existing distinction between oligarchs and democrats only worse.

The prospect of a short war.

Both parties were convinced that the war would not last long. Athens realised that her land army was no party for the feared Spartan phalanx, and their allies from Boeotia, but her navy was unmatched. With this in their mind they came to a surprising tactic: Attica would be left to the enemy, while all its inhabitants would be evacuated to the city where the fleet would import all the food needed. Where possible would the navy attack and damage the enemy as much as possible. The presence of the Athenian navy so close to the Peloponesse might also give the helots the courage to revolt once again against Sparta.

All Athens needed to do to win the war was to survive it. She had the financial resources of the empire to pay for any disciplining of her allies which might be necessary to ensure that essential supplies kept coming through. Athens and the harbour Piraeus were transformed into a huge fortress, and there was no need to leave it. Sparta's position was less comfortable as it did not have the tribute or any reserves. The only way to get her human resources was from her allies, and for this she had to satisfy their political and military needs. On top of this all was Sparta forced to fight an offensive war, as she had claimed to enter the war because she wanted to liberate Hellas from Athenian imperialism. Liberation meant taking positive steps to dismantle the Athenian empire.