Know your enemy and know yourself, and you will win a hundred battles.

Not quite. The thought is Sun Tzu's, and the wording in circulation is a misquote of chapter III, section 18. The Chinese is 知彼知己，百战不殆. Literally: know the other, know yourself, a hundred battles, no danger. The character 殆 means peril. It does not mean defeat, and it does not mean victory. Nothing in the line promises that you will win a hundred battles. The promise is safety, not a perfect record of wins.

Giles keeps the caution, and then finishes the lesson the shortened line drops. If you know the enemy and know yourself, you need 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. If you know neither the enemy nor yourself, you will succumb in every battle. Cite the whole of III.18. "A hundred wins" was never the sentence, and the two failures are part of the point.