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.
But though the decision was made it was impossible for them to take up arms at once, as they were unprepared; it was determined, however, that the several states should make the fitting preparations and that there should be no delay. Nevertheless, in providing themselves with what was needed there was spent, not indeed a full year, but somewhat less, before they invaded Attica and took up the war openly.
During this interval they kept sending embassies to the Athenians and making complaints, that they might have as good a pretext as possible for making war, in case the Athenians should refuse to consider them.