Know the enemy, know yourself, and in every battle you will be victorious.

解释 ( paraphrase )
知道自己的情况也知道对方的情况，哪怕一百次作战也能取得一百次胜利。
这条谚语告诉我们：既要了解自己又要了解对方，发挥自己的长处，攻击对方的短处，这样才能有把握取得胜利。

This proverb advises: Know both yourself and your opponent in order to enhance your strong points and attack his weak points. That is the secret of victory.

彼 (bǐ) : 指对方（敌方）
             The other party ( here : one's opponent ).

同义词 ( synonym )                                                                                                     知己知彼，百战不殆

造句  ( example )                                                                                                       只有了解掌握河势所要发生变化，坚持“以防为主，防重于抢，抢早抢小”的抢护原则，才能够做到知己知彼，百战百胜。