知己知彼

to know the enemy and know oneself (idiom, from Sunzi's "The Art of War")

know yourself and know your enemy, and you will never be defeated (idiom, from Sunzi's "The Art of War" 孙子兵法)

knowing the enemy and yourself will get you unscathed through a hundred battles (idiom, from Sunzi's "The Art of War")

those who understand others are clever, but those who know themselves are truly wise (idiom, from Laozi's 道德经

knowing oneself (idiom); self-knowledge

to catch an enemy off guard with a surprise attack (idiom, from Sunzi's "The Art of War" 孙子兵法)