The Peloponessian War
Book 1—Paragraph 23 by Thucydides

War began when the Athenians and the Peloponnesians broke the Thirty Years Truce which had been made after the capture of Euboea. As to the reasons why they broke the truce, I propose first to give an account of the causes of complaint which they had against each other and of the specific instances where their interests clashed: this is in order that there should be no doubt in anyone's mind about what led to this great war falling upon the Hellenes. But the real reason for the war is, in my opinion, most likely to be disguised by such an argument. What made war inevitable was the growth of Athenian power and the fear which this caused in Sparta. As for the reasons for breaking the truce and declaring war which were openly expressed by each side, they are as follows......