History of the Peloponnesian War

And the war began when the Athenians and Peloponnesians broke the thirty years' truce,[*](445 B.C.; cf. 115.1) concluded between them after the capture of Euboea.

The reasons why they broke it and the grounds of their quarrel I have first set forth, that no one may ever have to inquire for what cause the Hellenes became involved in so great a war.

The truest explanation, although it has been the least often advanced, I believe to have been the growth of the Athenians to greatness, which brought fear to the Lacedaemonians and forced them to war. But the reasons publicly alleged on either side which led them to break the truce and involved them in the war were as follows.