“Do I not destroy my enemies when I make them my friends?”
His response when "accused of treating his opponents with too much courtesy and kindness, and when it was pointed out to him that his whole duty was to destroy them", as quoted in More New Testament Words (1958) by William Barclay; either this anecdote or Lincoln's reply may have been adapted from a reply attributed to Holy Roman Emperor Sigismund:
:* Some courtiers reproached the Emperor Sigismond that, instead of destroying his conquered foes, he admitted them to favour. “Do I not,” replied the illustrious monarch, “effectually destroy my enemies, when I make them my friends?”
::* "Daily Facts" in The Family Magazine Vol. IV (1837), p. 123 http://books.google.de/books?id=aW0EAAAAQAAJ&pg=PA123&dq=destroy; also quoted as simply in "Do I not effectually destroy my enemies, in making them my friends?" in The Sociable Story-teller (1846)
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Abraham Lincoln 618
16th President of the United States 1809–1865Related quotes

“The enemy of my enemy is my friend, so that makes Google my best friend.”
Quoted in Miguel Helft, " Google and Salesforce Join to Fight Microsoft http://www.nytimes.com/2008/04/14/technology/14google.html?_r=1&oref=slogin", New York Times (April 14, 2008).
“Other dogs bite their enemies, but I my friends in order to save them.”
iii. 13. 44
Quotes by and about Diogenes

GG Allin on The Jerry Springer Show, May 5. 1993.
On The Jerry Springer Show

“I love my enemies, but am hell on my friends.”
[The Book of Ammon, 1970, Hennacy, 205]

Source: Quotes of Salvador Dali, 1931 - 1940, My Pictorial Struggle', S. Dali, 1935, Chapter: 'My Pictorial Struggle', p. 11

1840s, Essays: First Series (1841), Friendship

“If the enemy of my enemy is my friend, then surely you should be friend to my friend.”
Source: The Coldest Girl in Coldtown

http://www.telegraph.co.uk/sport/main.jhtml;jsessionid=CEDUJVE3P05PLQFIQMFSFFOAVCBQ0IV0?xml=/sport/2002/05/01/sotys02.xml&page=2
On himself

“Other dogs bite only their enemies, whereas I bite also my friends in order to save them.”
Stobaeus, iii. 13. 44
Quoted by Stobaeus