Reasons for the war
Corinth having now lost their influence over the western Aegian region, mobilised the city for war. Using all their energies to rebuid their humbled fleet and calling on all their allies for support for the war that was about to ensure. Bounties were also proclaimed throughout Greece for aid to help them against Corcyra.
Over the next year they spent their energies in regaining their prestigious fleet and arming their soliders.
Corcyra getting wind of what was about to happen to them sent ambassadors to Athens for aid. In the Athenian assembley they heard first from Corcyra and then from Corinth. At first the Athenians were inclined to help Corinth, but in a subsequent meeting they said they would help defend Corcyra on the basis that the Athenian fleet would help out only if there was an invasion to happen. As Corcyra had no intention of striking at Corinth, the aid was really to defend Corcyra only.
The swap of Athens from the first meeting, to help Corinth; as apposed to the second meeting, to help Corcyra; is an important one. As the animosity between these two cities would lead to the Peloponnesian War, the swap of sides helps us to understand why the Peloponnesians later on declare war against Athens.
In the first meeting it would have seemed obvious to the Athenian assembly that it should help out Corinth. It was more of a threat to Athens and a much closer neighbour. It would be worthwhile to be allies with them, especially as Sparta was beyond their borders and they held the very important postion of gatekeepers to the Peloponnese, it would be a accomplishment to have them on friendly terms.
By the second meeting the tide had turned and Athens thought it more worthwhile to help Corcyra for the following reasons. They held the dominent trade with the west and did have the potential to overthrow Corinth as a powerhouse. But most importantly if Athens helped out Corcyra Athens had the potential of capturing the trade to the west for themselves. Or letting the two cities wipe each other out, Athens could step right in over the top of them both and have access to the west without their help, so long as Corinth didn't capture Corcyra in the ensuring battle, thus the orders to the Athenian navy. This is also why Thucydides says in the Athenian assembly it was stated that "if Corcyra and Corinth were allowed to greatley diminish themselves on each other, it would not be a bad thing", not only because they thought a war with Sparta was on the cards but also because it allowed Athens the trade route with the west. Sparta did not have a fleet to fill in the void left by the two largest fleets in the Peloponnese, and regadless of that fact they did not like foreigners nor trade with them. Athens had a clear cut way of expanding their empire with minimal work.
In the near future this fact would not be overlooked by Sparta or the rest of the Peloponnese. But unlike Thucydides claim that the 'real' reason for the Peloponnesian War was because of Athens inclination to expand their empire. It seems more proper to say that the 'real' reason was because Athens started to make inroads against Peloponnesian trade (See Note#1), if this was allowed to continue they would have leverage on politics in the area and ultimatley be able to starve off aid to Sparta from other Peloponnesian cities. The real strength of Sparta was in the fact that the Peloponnesians would rally behind them in times of war. If Athens took control of the western trade and allowed democracies to flourish on the route, they would have political means of getting Peloponnesian cities to rebel against Sparta. Athens still had other ways of expanding their empire without venturing through the Peloponnese, say trading more with the north or other countries in the Mediterranian, they would have expanded their empire and not meddled in the Peloponnese giving Sparta no threat to go to war with them.
Proof of this can be found that when Sparta and Athens took care of the Persians and inherited the islands and the cities on the coast of Asia Minor, Sparta pretty much gave all this terriory to Athens on a plate, saying that while they fought to free the cities from Persian control, they did not want to both look after them nor defend them. Athens by default had a vast amount of cities to pay them tribute and the Athenian Empire had its foundations. So, if Sparta gave all these cities to Athens freely, they would not have worried about Athens expanding further. Sparta only wanted Athens not to interferre with the Peloponnese. But the oppotunity came for Athens to expand further through the Peloponnese, and they took the gamble; at best two Peloponnesian forces humble each other and the oppotunity presents itself to Athens to trade with the west, or at worst the ensuring battle engulfs the rest of Greece in a 30-year grueling war.
'The history of the Peloponnesian Wars' by Thucydidies (written c431 B.C),translated by Richard Crawley 1910.