“He will win who knows when to fight and when not to fight.”
Judgment includes restraint: choosing the wrong contest can be as damaging as fighting badly once the contest begins.
Source details
The Art of War, Chapter III “Attack by Stratagem,” section 17, Lionel Giles translation (1910), printed page 24 / scan leaf 80 in the original Giles edition on Wikisource, cross-checked against Project Gutenberg #17405. “He will win who knows when to fight and when not to fight” is the first of five listed military essentials for victory; it remains battlefield-strategy judgment rather than a context-free maxim about every disagreement.