The Art of War by active 6th century B.C. Sunzi

not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. [Li Ch’uan cites the case of Fu Chien, prince of Ch’in, who in 383 A.D. marched with a vast army against the Chin Emperor. When warned not to despise an enemy who could command the services of such men as Hsieh An and Huan Ch’ung, he boastfully replied: "I have the population of eight provinces at my back, infantry and horsemen to the number of one million; why, they could dam up the Yangtsze River itself by merely throwing their whips into the stream. What danger have I to fear?" Nevertheless, his forces were soon after disastrously routed at the Fei River, and he was obliged to beat a hasty retreat.] If you know neither the enemy nor yourself, you will succumb in every battle. [Chang Yu said: "Knowing the enemy enables you to take the offensive, knowing yourself enables you to stand on the defensive." He adds: "Attack is the secret of defence; defence is the planning of an attack." It would be hard to find a better epitome of the root-principle of war.]