Why did Athens and Sparta come to blows in the Great Peloponnesian War?
Conflict between the various Greek city states was depressingly frequent at this time. Despite the existence of a common language and a number of cultural similarities, war broke out with alarming regularity, emphasizing the primarily territorial nature of such conflict. The Athenians and the Spartans had actually fought together during the Persian War, but once their common enemy had been defeated a bitter rivalry soon developed between the former allies. Sparta was worried that Athens was becoming too powerful and would one day come to threaten Sparta directly.
This was a perfectly legitimate fear. The Athenian empire was growing rapidly, and with its system of democracy and greater wealth, Athens was a more attractive strategic ally for smaller Greek city states in need of protection. One such state was Megara. Athens concluded an alliance with Megara, an ally of Sparta, allowing it to gain an important strategic foothold on the isthmus of Corinth. The Spartans felt threatened by this development, and their increasing fears of territorial expansion by the Athenians led directly to the First Peloponnesian War.
The ultimate cause of the Peloponnesian War was the fact that Athens and Sparta were the two "superpowers" of contemporary Greece. Given their rivalry, it was essentially inevitable that they would "come to blows."
The more proximate cause of the war was conflict between Athens and two of Sparta's allies, Corinth and Megara. As Corinth and Megara came into conflict with Athens, they called on Sparta for help. They even threatened to withdraw from the Peloponnesian League (headed by Sparta) if it did not help them. To protect its alliances, Sparta joined the war against Athens.