“You've got to take it on faith that the enemy of your enemy is your friend.”

Source: The Boyfriend List: 15 Guys, 11 Shrink Appointments, 4 Ceramic Frogs and Me, Ruby Oliver

Source: Lyonesse Trilogy (1983-1989), The Green Pearl (1985), Chapter 8, section 3 (p. 480)

Source: The Goblin Quest Series, Goblin Hero (2007), Chapter 7 (p. 117)

Machiavelli commented on the relative ease of gaining favor from friends and enemies in Chapter 20 of The Prince, quoted above. However, this particular wording comes from a line spoken by Michael Corleone in The Godfather Part II (1974), written by Mario Puzo and Francis Ford Coppola:
My father taught me many things here. He taught me in this room. He taught me: keep your friends close, but your enemies closer.
Misattributed

This has often been attributed to Sun Tzu and sometimes to Petrarch. It comes most directly from a line spoken by Michael Corleone in The Godfather Part II (1974), written by Mario Puzo and Francis Ford Coppola:
My father taught me many things here. He taught me in this room. He taught me: keep your friends close but your enemies closer.
Niccolò Machiavelli, who is also sometimes credited, wrote on the subject in The Prince:
It is easier for the prince to make friends of those men who were contented under the former government, and are therefore his enemies, than of those who, being discontented with it, were favourable to him and encouraged him to seize it.
Misattributed

Source: The Lying Game

Diary of an Unknown (1988)