History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
for they were now becoming individually less keenly sensible of their private griefs, and as to the needs of the state as a whole they esteemed him invaluable.
For so long as he presided over the affairs of the state in time of peace he pursued a moderate policy and kept the city in safety, and it was under him that Athens reached the height of her greatness; and, after the war began, here too he appears to have made a farsighted estimate of her strength. Pericles lived two years and six months beyond the beginning of the war;
and after his death his foresight as to the war was still more fully recognized.
For he had told the Athenians that if they would maintain a defensive policy, attend to their navy, and not seek to extend their sway during the war, or do anything to imperil the existence of the state, they would prove superior. But they not only acted contrary to his advice in all these things, but also in matters that apparently had no connection with the war they were led by private ambition and private greed to adopt policies which proved injurious both as to themselves and their allies; for these policies, so long as they were successful, merely brought honour or profit to individual citizens, but when they failed proved detrimental to the state in the conduct of the war.[*](The reference is especially to the Sicilian expedition; the pernicious results were seen in the Decelean war.)