“That which is good for the enemy harms you, and that which is good for you harms the enemy.”
Quello che giova al nimico nuoce a te, e quel che giova a te nuoce al nimico.
Rule 1 from Machiavelli's Lord Fabrizio Colonna: libro settimo (Book 7) http://babel.hathitrust.org/cgi/pt?id=njp.32101013672561;view=1up;seq=176 (Modern Italian uses nemico instead of nimico.)
The Art of War (1520)
Original
Quello che giova al nimico nuoce a te, e quel che giova a te nuoce al nimico.
The Art of War (1520)
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Niccolo Machiavelli 130
Italian politician, Writer and Author 1469–1527Related quotes

“Your worst enemy cannot harm you as much as your own unguarded thoughts.”

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14 June 1853
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“No tool is more beneficial than intelligence. No enemy is more harmful than ignorance.”
http://encarta.msn.com/quote_561557126/Intelligence_No_tool_is_more_beneficial_than.html
The Book of Guidance into the Lives of the Twelve Imams, 1981, "The Life of the Commander of the Faithful" (I. K. A. Howard (tr.)).

1950s, Loving Your Enemies (November 1957)

“A man's enemies have no power to harm him, if he is true to himself and loyal to God.”
Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 208.