Final answer:
The Peloponnesian War was caused by the rivalry between Athens and Sparta, and it led to a shift in power, weakened social and political order, and transformed Sparta.
Step-by-step explanation:
The Peloponnesian War was caused by the rivalry between Athens and Sparta, two major city-states in ancient Greece. Athens, possessing a powerful navy and empire, threatened the dominance of Sparta and its allies. In response, Sparta formed the Peloponnesian League and declared war against Athens.
The war had several significant effects on ancient Greece. It led to a shift in power from Athens to Sparta and the decline of the Athenian empire. The war also weakened the social and political order of ancient Greece and resulted in a more cosmopolitan outlook among the Greeks.
Another effect of the war was the transformation of Sparta itself. It became a naval power and had to adapt its military and political strategies. The war also undermined Sparta's cultural xenophobia and austerity, leading to its defeat by the polis of Thebes.