彼を知り己を知れば、百戦殆からず
Idiom- 1if you know your enemy and know yourself, in a hundred battles you will never be defeated ; know your enemy Translated by Jiyomi
Other writings and readings
彼を知り己を知れば百戦殆からず
expressions (phrases, clauses, etc.)
JMdict entry 2429140. Source and licence.