Athens and Sparta, having fought together previously against the Persians, have now come to the point where Sparta thinks Athens and their Delian League have become too powerful. Sparta develops the Peloponnesian League and begins what is known as the Peloponnesian War (431-404 BC.)
This war has been divided by historians into three main phases. In the first phase, Athens uses their vast naval resources to attack the coast of the Peloponnese, trying to keep the unrest at bay. This was in response to Sparta attacking Attica over and over. The first phase ended with the Peace of Nicias being signed in 412 BC.
However, fighting flared up again in the Peloponnese and the treaty became useless. Athens launched a huge force of power against Syracuse in Sicily. That was a complete disaster with the entire force dying in the attempt in 413 BC.
The final phase has two common names – the Decelean War or the Ionian War. Persia joined Sparta and with the rebellion that had formed in Athens, was able to get the upper hand against Athens and their naval resources, squelching them and finding victory when Athens surrendered the next year.
Corinth and Thebes wanted Athens destroyed and all their citizens to be slaves, but Sparta rejected that idea.
The Peloponnesian War changed the face of the ancient Greek world. Athens, the once superpower, was taken down and was no longer a powerhouse. Sparta was established as the leader. The costly war wasn’t just costly for Athens and Sparta – all of Greece bore the cost, bringing poverty across all the city-states.
This war was not a success for the victor or the loser. Civil war became common across the city-states post-war. During this time, neighboring Macedonia grew in military strength and would go on to conquer the majority of the Greece peninsula. The Peloponnesian War was the end of the golden age of Greece.