War, Napoleon Bonaparte

Never interrupt your enemy when he is making a mistake.

- Napoleon Bonaparte (1769–1821)

Friend, Arabic Proverb

Any wise enemy is better than an ignorant friend.

- Arabic proverb

Quoted in Carol Bardenstein, Translation and Transformation in Modern Arabic Literature:The Indigenous Assertions of Muḥammad 'Uthmān Jalāl, Otto Harrassowitz Verlag, 2005, p. 66.

Arabic proverb: Proverbs from all Arabic speaking parts of the world.

http://en.wikiquote.org/wiki/Arabic_proverbs
http://en.wikiquote.org/wiki/Category:Proverbs

Foe, Turkish Proverb

Be your enemy an ant, see in him an elephant.

- Turkish Proverb

Friend, Russian Proverb

An enemy will agree, but a friend will argue.

- Russian Proverb

AFI #058 Al Pacino as Michael Corleone from The Godfather Part II (1974)

There are many things my father taught me here in this room. He taught me: keep your friends close, but your enemies closer.

- Al Pacino as Michael Corleone from The Godfather Part II (1974)

The bolded line is ranked #58 in the American Film Institute's list of the top 100 movie quotations in American cinema.