知己知彼 Know yourself, know your enemy
Explanation
这个成语出自《孙子兵法》，意思是说，如果对敌我双方的情况都能了解透彻，打起仗来就可以立于不败之地。现在也常用来比喻在做任何事情之前，都要先了解自己的情况和对手的情况，才能做到有的放矢，取得成功。
This idiom comes from Sun Tzu's "The Art of War," meaning that if one thoroughly understands both their own and their enemy's situations, they can be undefeated in battle. Now, it is also often used to illustrate that before doing anything, one should first understand their own situation and that of their opponent, so that they can act purposefully and achieve success.
Origin Story
春秋时期，吴国和越国经常发生战争，吴王阖闾派伍子胥去攻打越国，结果大败而归。阖闾很不高兴，问伍子胥失败的原因。伍子胥说：“我们对越国的情况不了解，所以才会打败仗。”阖闾听后很后悔，便让伍子胥认真研究越国的形势，并派他再次攻打越国。这次，伍子胥在充分了解越国情况的基础上，制定了周密的作战计划，最后取得了胜利。这个故事告诉我们，只有知己知彼，才能百战不殆。
During the Spring and Autumn Period, the Kingdom of Wu and the Kingdom of Yue frequently waged war against each other. King Helü of Wu dispatched Wu Zixu to attack Yue, but he returned in a crushing defeat. King Helü was displeased, and questioned Wu Zixu about the reasons for failure. Wu Zixu replied, "We lacked a thorough understanding of Yue's situation; therefore, we suffered defeat." King Helü, filled with regret, instructed Wu Zixu to carefully study Yue's circumstances. He then sent Wu Zixu to attack Yue again. This time, after comprehensively assessing Yue's condition, Wu Zixu formulated a well-defined battle plan and eventually secured victory. This tale highlights the importance of knowing oneself and one's opponent for achieving invincibility in battle.
Usage
常用于军事、政治、商业等领域，形容在竞争中，要充分了解自己和对手的情况，才能立于不败之地。
Often used in the fields of military, politics, and business to describe that in competition, one must fully understand their own situation and their opponent's situation in order to stand undefeated.
Examples
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商场如战场，知己知彼，百战不殆。
shang chang ru zhan chang,zhi ji zhi bi,bai zhan bu dai
The market is like a battlefield; knowing yourself and your opponent is crucial for success.
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要打胜仗，必须知己知彼，才能制定出有效的作战方案。
yao da sheng zhang,bi xu zhi ji zhi bi,cai neng zhi ding chu you xiao de zuo zhan fang an
To win a battle, you must know yourself and your enemy to formulate effective combat plans.