In classical Greece after development of Athens Empire all other Greek polis had come to the point of fear and jealousy especially Sparta and Corinth. They felt threatened by rising power of Athens. So the Civil War could not be escaped.

The war was slow to develop.

Let me tell you some of the reasons why Athens engaged into war. It is very simple. Money. Athenian empire had money flowing in from all over its vest territories in forms of tribute. Athens fleet needed large amounts of money to maintain it and to pay salaries. And the fleet was a big chunk of the power the city had. New ships needed to be added.

Most of the buildings that were build at that time were financed from the money coming in as a tribute. Just recall Parthenon. Also Athens as a center of the empire was always full of visitors( some coming to the courts to settle their disputes, some for commerce and some came to enjoy the beauty of the town) who spent money on food, housing, pleasures etc. That provided jobs for many Athenians.

Another big reason would be that Athens largely depended on the import of grain( most of which came from Black sea). Control of the route from Hellespont to Athens was vital. And you can see, later on during the war that the only thing ending the war was when Sparta and its allies were able to put a lock on the grain road to the Athens. Farther down into the page you would be able to see that possession of this water way was a reason for almost all battles at the end of the war.

Perhaps if we were to look at Athens republic we would notice that the hole regimes well being rested on the money coming from the empire. Most off the people were well off because government was able to aid the poor, provide food in hard times.Athenians were able to spent their time on the government duties because their were paid for it( which is very unusual for that time since non of the sides had gathered such wealth). Because people didn't have to be rich to govern it provided the base for democracy of which Athenins were very proud.

So as you can see Athens couldn't let the empire fall apart because their own well being was dependent on the countinue of the empire.

The rest of the ancient Greece was in jealousy and fear of the Athens. Sparta-Athens long enemy was seeing a threat to its powers seens Athens tried to move closer and closer into Peloponesus.The war was unavoidable.

So the Affair of Epidamnus and the Affair of Potidaea were on their way. First one was when little city of Epidamnus was seeking help from Corcyra, didn't receive none and went to Corinth. Corcyra saw a threat when Corinthians put their solders in the Epidamnus and tried to go to war, but saw itself alone in circle of enemies so Corcyra wanted to join Athenian empire which it did. The war started.But Athenians defended like that"Neither are beginning the war, Peloponnesians, nor are we breaking the treaty, but those Corcyraens are our allies, and we come to help them. So if you want to sail anywhere else, we place no obstacles on your way, but if you are going to sail against Corcyra, or any other of her possessions, we shall our best to stop you."1.