KING : Am I the strongest or am I not?
BECKET: You are, today. But one must never drive one’s enemy to despair. It makes him strong. Gentleness is better politics. It saps virility. A good occupational force must never crush, it must corrupt.

Jean Anouilh (1910-1987) French dramatist

Becket, Act 2 (1959) [tr. Hill (1961)
    (Source)

The lines remain intact in Edward Anhalt's 1964 screenplay.