A society of sheep must in time beget a government of wolves.

bertrand de jouvenel
Bertrand de Jouvenel (1903–1987) French philosopher, political economist, futurologist
(Attributed)

Sometimes attributed to his On Power (1949), but not found there.

The quotation is well-established in 1949, cited in the Congressional Record (1949-06-15) and Reader's Digest (1949-05).

Variants are sometimes attributed to Edward R. Murrow, withiout citation, including "A nation of sheep will beget a government of wolves" and "A nation of sheep will soon have a government of wolves."

I have also seen the quote (and variants) misattributed to Agatha Christie (including to her memoir An Autobiography (1977), which does not have the text. This connection may be because SF author J. T. McIntosh quotes "A nation of sheep will beget a government of wolves" in his novella (1955-09), "The Man Who Cried 'Sheep!'" ch. 5, Magazine of Fantasy and Science Fiction, Vol. 9, No. 3. Christie's 1925 fantasy "The Fourth Man" also appears in that same issue of MFSF, two page after the quote in McIntosh's story.