To put it simply, know your enemy.
Verses from the book occur in modern daily Chinese idioms and phrases, such as the last verse of Chapter 3:
So it is said that if you know your enemies and know yourself, you will fight without danger in battles.If you only know yourself, but not your opponent, you may win or may lose.If you know neither yourself nor your enemy, you will always endanger yourself.
This has been more tersely interpreted and condensed into the modern proverb:
知己知彼 百戰不殆 （知彼知己，百战不殆）
If you know both yourself and your enemy, you can come out of hundreds of battles without danger.
Many people interpret this sentence as 'If you know both sides, you will win a hundred times in one hundred battles. (知己知彼 百戰百勝)'. This translation is incorrect. The word '殆' in Chinese means 'danger'. '百' in this sentence is better interpreted as 'numerous' rather than 'hundred'.
Furthermore, knowing both sides doesn't guarantee winning. '知己知彼 百戰百勝' is untrue since in the beginning paragraph of chapter four, Sun Tsu wrote 'Hence, we can well predict who would win but there is no strategy guaranteeing winning (故曰: 勝可知，而不可為。)'. The reason of the uncertainty is quite simple. How about dealing with the opponent who knows both sides better than you do?
Similar verses have also been borrowed -- in a manner construing skillfulness as victory "without fighting" -- for example:
Therefore one hundred victories in one hundred battles is not the most skillful. Seizing the enemy without fighting is the most skillful.
And, the most famous quotation (chapter 1, paragraph 18):
All warfare is based on deception.
And, as we learned about Purim, the enemy, like G-d, may be hidden, disguised.