The main cause of the Peloponnesian War, according to Thucydides, was the growth of Athenian power. After the end of the Persian wars, Athens became the rival of Sparta, the former great power in the region. As Athens's power grew greater and greater, it came to dominate the city-states it had been in alliance with and turned them into client states that had to pay Athens tribute. This was considered Athens's golden age, the period when wealth poured into the city, and the arts and philosophy flourished. It was the age of Socrates and Plato.
Sparta, used to being the dominant power in the region, became increasingly worried about Athens's growing strength. Although Sparta had previously tried to sidestep war, a war party grew increasingly dominant and was encouraged by Sparta's allies, such as Corinth. Eventually, in 432 BC, Sparta did declare war on Athens, fearing that if it did not strike soon, Athenian strength would soon grow too great to challenge and Sparta and it allies would find themselves under Athens's thumb.
The outcome of the war was a victory for Sparta. It became the dominant power in the region, and Athens became its subject state. However, the Peloponnesian War in reality benefitted nobody. It was extremely expensive and left all the Greek states impoverished. The war also changed the nature of the relations between the various Greek states, with total war bent on the utter destruction of rivals replacing the limited warfare the Greek had waged earlier.