expression, suru verb (precursor)
strategies must be kept secret to succeed
A classical proverb emphasizing that plans and strategies are best kept confidential; secrecy is essential for success. The phrasing uses classical grammar (なる attributive form of なり, and す as precursor to modern する).
「謀は密なるを良しとす」という言葉があるように、作戦は秘密にしておくべきだ。
As the saying goes, 'strategies cannot succeed unless they are kept secret,' so you should keep your plans confidential.