I first understood the power of the documentary during the editing of my first film, The Quiet Mutiny.
In the commentary, I make reference to a chicken, which my crew and I encountered while on patrol with American soldiers in Vietnam.
"It must be a Vietcong chicken -- a communist chicken," said the sergeant. He wrote in his report: "enemy sighted.".
The chicken moment seemed to underline the farce of the war -- so I included it in the film.
That may have been unwise.
The regulator of commercial television in Britain -- then the Independent Television Authority or ITA -- had demanded to see my script.
What was my source for the political affiliation of the chicken? I was asked. Was it really a communist chicken, or could it have been a pro-American chicken?
Of course, this nonsense had a serious purpose; when The Quiet Mutiny was broadcast by ITV in 1970, the US ambassador to Britain, Walter Annenberg, a personal friend of President Richard Nixon, complained to the ITA.
He complained not about the chicken but about the whole film. "I intend to inform the White House," the ambassador wrote. Gosh.
The Quiet Mutiny had revealed that the US army in Vietnam was tearing itself apart. There was open rebellion: drafted men were refusing orders and shooting their officers in the back or "fragging" them with grenades as they slept.
None of this had been news. What it meant was that the war was lost; and the messenger was not appreciated.
The Director-General of the ITA was Sir Robert Fraser. He summoned Denis Foreman, then Director of Programmes at Granada TV, and went into a state of apoplexy. Spraying expletives, Sir Robert described me as a "dangerous subversive."
What concerned the regulator and the ambassador was the power of a single documentary film: the power of its facts and witnesses: especially young soldiers speaking the truth and treated sympathetically by the film-maker.
I was a newspaper journalist. I had never made a film before and I was indebted to Charles Denton, a renegade producer from the BBC, who taught me that facts and evidence told straight to the camera and to the audience could indeed be subversive.
This subversion of official lies is the power of documentary. I have now made 60 films and I believe there is nothing like this power in any other medium.
In the 1960s, a brilliant young film-maker, Peter Watkins, made The War Game for the BBC. Watkins reconstructed the aftermath of a nuclear attack on London.