Laudator Temporis Acti

Polybius 2.35.5-9 (tr. Ian Scott-Kilvert):

For I believe that it is the proper function of history to hand down to posterity such episodes in the drama of Fortune, so that our successors may not through sheer ignorance of the facts be overcome by terror at these sudden and unexpected incursions of the barbarians, but should understand how short-lived and easily extinguished such movements may prove to be. If they are fortified with this knowledge they can face the invader and try their prospects of safety to the very limit before they yield an inch of their most vital interests.

Indeed I consider that those writers who recorded and handed down to us the story of the invasion of Greece by the Persians and the attack by the Gauls on Delphi made a great contribution to the Greek peoples' fight to preserve their common liberty. For there is no reason why the enemy's superiority in numbers or in weapons or supplies need terrify a man into abandoning his ultimate hope — that is to fight to the last for his native land — so long as he keeps steadily in view the knowledge of how great a part the unexpected has often played in these campaigns, and remembers how many myriads of troops, what immense armaments and what overweening confidence have been defeated by the resolution and the ability of men who faced the danger with intelligence and cool calculation.