“Choose your enemies carefully: you'll probably be known much better and far longer for who they were, than for anything else you ever managed to accomplish.”

Cedars Of Lebanon
Lyrics, No Line On The Horizon (2009)

Advice to the Youth of Mississippi (31 December 1964) http://www.britannica.com/blackhistory/article-9399834
Variant: You get freedom by letting your enemy know that you'll do anything to get your freedom; then you'll get it. It's the only way you'll get it.
Context: You get freedom by letting your enemy know that you'll do anything to get your freedom; then you'll get it. It's the only way you'll get it.

Part Eight “The Return”, Chapter v “Nonesuch”, Section 2 (p. 353)
(1987), BOOK TWO: THE FUGUE

Source: Letters of Katherine Anne Porter

Source: Eric

Book 7; Variant translation: No enterprise is more likely to succeed than one concealed from the enemy until it is ripe for execution.
Nothing is of greater importance in time of war than in knowing how to make the best use of a fair opportunity when it is offered.
Few men are brave by nature, but good discipline and experience make many so.
Good order and discipline in an army are more to be depended upon than ferocity.
As translated by Neal Wood (1965)
The Art of War (1520)
Context: No proceeding is better than that which you have concealed from the enemy until the time you have executed it. To know how to recognize an opportunity in war, and take it, benefits you more than anything else. Nature creates few men brave, industry and training makes many. Discipline in war counts more than fury.

Source: Survivor

Original: Scegli con cura solo chi osa affrontare la vita sempre al tuo fianco.
Source: prevale.net