Published as having been made in an interview with William Griffin, editor of theNew York Enquirer (August 1936), who was indicted for sedition by F.D.R.’s Attorney General Francis Biddle in 1942. In a sworn statement before Congress in 1939 Griffin affirmed Churchill had said this. Congressional Record (1939-10-21) vol. 84. p. 686. In 1942, Churchill admitted having had the 1936 interview but disavowed having made the statement (The New York Times, 1942-10-22. p. 13).
In his article “The Hidden Tyranny,” Benjamin Freedman atrributed this quotation to an article in the non-interventionist publication Scribner’s Commentator in 1936. However, that magazine did not exist until 1939. He probably got the date wrong or may have been referring to one of its predecessor Scribner’s Monthly or Payson Publishing’s The Commentator.