The fate of those of their neighbours who had already rebelled and had been subdued was no lesson to them; their own prosperity could not dissuade them from affronting danger; but blindly confident in the future, and full of hopes beyond their power though not beyond their ambition, they declared war and made their decision to prefer might to right, their attack being determined not by provocation but by the moment which seemed propitious. The truth is that great good fortune coming suddenly and unexpectedly tends to make a people insolent; in most cases it is safer for mankind to have success in reason than out of reason; and it is easier for them, one may say, to stave off adversity than to preserve prosperity.

Book III, 3.39- - History of the Peloponnesian War