One of the main causes of the Peloponnesian War was the fact that Sparta had alliances with the Argos and Achaea cultures. Alliance is a loose term at this point – these alliances were founded soles on trade and weak treaties. Sparta was known as the military capital of the Greek world. After the Persian
War, Athens was getting attention from the Delian League. Sparta felt that Athens was stealing Sparta’s military glory.
Later on, Athens decided to invade Thasos. Sparta was supposed to aid in this invasion, however, Sparta suffered a natural disaster and was unable to come to Athens’ aid. Then, Athens tried to aid Sparta with rebuilding the city after their disaster, however, Sparta was rude and pretty much told Athens to leave them alone. Because this is Sparta we are talking about, and they have no sense of manners, they were rude in telling Athens to leave, and in turn, Athens was aggravated by how they were treated.
The Peloponnesian War started because of a bunch of little things that kept piling up, and finally Sparta had enough and went to war. During the war, Sparta basically destroyed Athens in battles, but when Persia finally intervened, the war ended with Sparta emerging as the initial victor, yet Sparta collapsed under the pressure of being the winner.
This is similar to how siblings act. The siblings alternate in who annoys the other sibling, and finally when someone had enough and blows their top, there is a quarrel. Then during the arguments and fights, the parent or guardian figure steps in to stop the fighting. This is was just on a very large scale, and lasting almost 30 years.