Let him remember that many before now have tried to chastise a wrongdoer, and failing to punish their enemy have not even saved themselves; while many who have trusted in force to gain an advantage, instead of gaining anything more, have been doomed to lose what they had. Vengeance is not necessarily successful because wrong has been done, or strength sure because it is confident; but the incalculable element in the future exercises the widest influence, and is the most treacherous, and yet in fact the most useful of all things, as it frightens us all equally, and thus makes us consider before attacking each other.
Book IV, 4.62-- - History of the Peloponnesian War
