Who fought in the Peloponnesian war? What was one cause of the war?
At the end of the Persian wars, Athens became extraordinarily powerful, looting the treasury of the Delian league for an extensive program of rebuilding and forcing was were almost equal allies to become subordinate to Athenian power. Sparta, fearing the growth of Athenian power, and smaller city states desiring to maintain their independence, ended up opposed to Athens.
In many ways the war was inevitable. Sparta was a land power and Athens a maritime one. Athens was a democracy and Sparta an oligarchy. Athens was the major power in mainland Greece with many island colonies and Sparta dominated the Peloponnesus.
Other participants in the wars included many of Sparta's mainland allies such as Corinth and Thebes, and Athens concluded alliance with Argos and Thessaly, but over the course of the wars Athenian imperial ambitions and arrogance tended to drive away allies.