彼を知り己を知れば百戦殆うからず
(Kare wo shiri onore wo shireba hyakusen ayaukarazu;
“When you know your opponent and yourself, you need not fear a hundred battles.”)

Definition:

If you know the strengths, weaknesses, and situation of yourself, your allies, and your foes, then you need never fear defeat even through a hundred battles. Information and awareness, especially self-awareness, are vital to success.

Breakdown:

We begin with a conditional clause, marked by and ending with the suffix ば (ba). The conditional suffix applies to two verb phrases, both using the particle を (wo) to mark a noun as the object of the verb 知る (shiru), “to know.” In the first phrase, the noun is 彼 (kare), often translated as “he” but in this case meaning “that person,” or in this context, one’s opponent in battle. In the second phrase, the noun is 己 (onore), “self.” In the first phrase, the verb is in conjunctive form, allowing it to connect to the second. And in the second, it’s in perfective form, connecting with a ば that is conditional rather than hypothetical – that is to say, in Japanese it’s a “when” ba rather than an “if” ba.

The latter part of the saying comprises the number 百 (hyaku), “hundred,” the noun 戦 (sen), “battle,” and the adverb 殆うく (ayauku), “barely,” “with danger,” in negative sentence-final form.

Notes:

殆 is no longer a standard character for “dangerous,” nor is ayauku a standard reading for this character. Although 危 is not in the original text, using it is apparently not considered an error in contemporary Japanese.

As you may well have expected, this saying comes straight from Sun Tzu’s The Art of War – specifically the third chapter, on “strategic attack” (謀攻).

Example sentence:

「彼を知り己を知れば百戦殆うからずというのに、あの政治屋はスリルのために戦争ごっこしたがってるみたいだな。人のことはおろか、己のことすら知らないくせに」

(“Kare wo shiri onore wo shireba hyakusen ayaukarazu to iu no ni, ano seijika wa suriru no tame ni sensou gokko shitagatteru mitai da na. Hito no koto wa oroka, onore no koto sura shiranai kuse ni.”)

[“They say that when you know your opponent and know yourself, you need not fear a hundred battles – but that politician seems to want to play at war just for the thrill. Even though – much less knowing others – he doesn’t even know himself.”]