People remember this line as a formula for victory. “Know the other and know yourself, and you will win a hundred out of a hundred.” Sun Tzu never said that.
知彼知己, 百戰不殆
zhi bi zhi ji, bai zhan bu dai
Know the other and know yourself, and a hundred battles will bring no danger. (The Art of War, Attack by Stratagem, translated directly)
What he promised was not victory. It was the absence of danger. Victory depends on the other side. Not being in danger depends on you. So this is not a sentence of confidence. It is a sentence of modesty.